1. A long-distance box for a Passive Optical Networks (PON) system in which different PONS coexist, wherein the long-distance box comprises a long-distance amplification box and a wavelength division multiplexing filter; the system in which the different PON systems coexist comprise a first PON system and a second PON system; the long-distance amplification box is configured to process uplink/downlink lights from PON systems, which passes through an optical path in which the long-distance amplification box is located, the long-distance amplification box comprises a first long-distance amplification box located in a first optical path and a second long-distance amplification box located in a second optical path; the wavelength division multiplexing filter comprises a first wavelength division multiplexing filter and a second wavelength division multiplexing filter; wherein, the first wavelength division multiplexing filter is configured to split the downlink lights and couple the uplink lights from different PON systems, the uplink/downlink light from the first PON system is transmitted through the first optical path and amplified by the first long-distance amplification box, and the uplink/downlink light from the second PON system is transmitted through the second optical path and amplified by the second long-distance amplification box; and the uplink lights transmitted through the first optical path and the second optical path are coupled to a trunk optical fiber through the first wavelength division multiplexing filter; and the second wavelength division multiplexing filter is configured to split the uplink lights and couple the downlink lights from different PON systems, the uplink/downlink light from the first PON system is transmitted through the first optical path and amplified by the first long-distance amplification box, and the uplink/downlink light from the second PON system is transmitted through the second optical path and amplified by the second long-distance amplification box; and the downlink lights transmitted through the first optical path and the second optical path are coupled to the trunk optical fiber through the second wavelength division multiplexing filter, wherein the wavelength division multiplexing filter consists of filtering sheets with double windows and comprises an interface R, an interface P and an interface C; the uplink/downlink light from the first PON system is reflected and input/output through the interface R, the uplink/downlink light from the second PON system is transmitted and input/output through the interface P; and the interface C is connected with the trunk optical fiber; the interface C of the first wavelength division multiplexing filter is connected with the trunk optical fiber at an Optical Line Terminal (OLT) side, the interface R of the first wavelength division multiplexing filter is connected with one end of the first long-distance amplification box in the first optical path, and the interface P of the first wavelength division multiplexing filter is connected with one end of the second long-distance amplification box in the second optical path; the interface C of the second wavelength division multiplexing filter is connected with the trunk optical fiber at an Optical Network Unit (ONU) side, the interface R of the second wavelength division multiplexing filter is connected with the other end of the first long-distance amplification box in the first optical path, and the interface P of the second wavelength division multiplexing filter is connected with the other end of the second long-distance amplification box in the second optical path, wherein, the downlink light from the first PON system is emitted by the OLT of the first PON system, transmitted to the interface C of the first wavelength division multiplexing filter through the trunk optical fiber, output to the first optical path through the interface R of the first wavelength division multiplexing filter, output to the interface R of the second wavelength division multiplexing filter by the first long-distance amplification box corresponding to the first PON system, then output to the trunk optical fiber through the interface C of the second wavelength division multiplexing filter, and finally transmitted to the ONU through a light splitter and a branch optical fiber; the uplink light from the first PON system is emitted by the ONU of the first PON system, transmitted to the interface C of the second wavelength division multiplexing filter through the branch optical fiber and the light splitter, output to the first optical path through the interface R of the second wavelength division multiplexing filter, output to the interface R of the first wavelength division multiplexing filter by the first long-distance amplification box corresponding to the first PON system, then output to the trunk optical fiber through the interface C of the first wavelength division multiplexing filter, and finally transmitted to the OLT of the first PON system; the downlink light from the second PON system is emitted by the OLT of the second PON system, transmitted to the interface C of the first wavelength division multiplexing filter through the trunk optical fiber, output to the second optical path through the interface P of the first wavelength division multiplexing filter, output to the interface P of the second wavelength division multiplexing filter by the second long-distance amplification box corresponding to the second PON system, then output to the trunk optical fiber through the interface C of the second wavelength division multiplexing filter, and finally transmitted to the ONU through the light splitter and the branch optical fiber; and the uplink light from the second PON system is emitted by the ONU of the second PON system, transmitted to the interface C of the second wavelength division multiplexing filter through the branch optical fiber and the light splitter, output to the second optical path through the interface P of the second wavelength division multiplexing filter, output to the interface P of the first wavelength division multiplexing filter by the second long-distance amplification box corresponding to the second PON system, then output to the trunk optical fiber through the interface C of the first wavelength division multiplexing filter, and finally transmitted to the OLT of the second PON system.