1. A method for building an MOS transistor model including a statistical variation of noise, comprising: measuring noise in MOS transistors from different dies; creating a noise distribution diagram in accordance with the obtained noise data of the MOS transistors; adding a variation of a noise parameter in at least one noise model file into a library file of an MOS transistor to simulate noise in MOS transistors; if a simulation result does not cover the noise data in the noise distribution diagram, continuously changing the variation of the noise parameter until the simulation result covers the noise data in the noise distribution diagram; and if the simulation result covers the noise data in the noise distribution diagram, adding a corresponding variation of the noise parameter into the library file of the MOS transistor as the MOS transistor model including a statistical variation of noise.