1. A low-orbit satellite-borne image-spectrum associated detection method, comprising: (1) detecting and tracking moving targets and dynamic phenomena using a low-orbit satellite, comprising: (1-1) performing background clutter suppression on an infrared image; (1-2) performing threshold segmentation on the image after the suppression, and extracting the suspected moving targets and dynamic phenomena; (1-3) taking the suspected moving targets and dynamic phenomena as processing objects, detecting the real moving targets and dynamic phenomena, and placing the center of field of view (FOV) at predicted positions of the moving targets or the dynamic phenomena so as to track the moving targets or the dynamic phenomena, the detecting including: (A1) performing pixel offset compensation of coordinate transformation on processing objects of multiple frames of images; (A2) performing correlation of the multiple frames of images after pixel offset compensation to detect the real processing objects; (A3) when the processing objects are moving targets, calculating image plane speed characteristics of the moving targets to obtain predicted positions thereof; and when the processing objects are dynamic phenomena, calculating gray characteristics of the dynamic phenomena to obtain predicted positions thereof; and (A4) positioning positions of next-frame processing objects to the predicted positions of the processing objects, and placing the center of FOV at the predicted positions of the processing objects; (1-4) measuring infrared broadband spectra of regions surrounding the center of FOV to obtain regional spectra corresponding to the moving targets or the dynamic phenomena; and (1-5) taking the moving targets or the dynamic phenomena tracked in Step (1-3) as processing objects, and repeating Steps (A1) to (A4) to track the moving targets or the dynamic phenomena, (2) performing multi-dimensional characteristic analysis on infrared spectra of the moving targets and the dynamic phenomena to identify the moving targets and the dynamic phenomena, comprising: (2-1) positioning the moving targets and the dynamic phenomena and measuring infrared spectra of the moving targets and the dynamic phenomena; (2-2) performing compensation and correction on the infrared spectra of the moving targets and the dynamic phenomena; (2-3) removing the regional spectra corresponding to the moving targets and the dynamic phenomena from the infrared spectra of the moving targets and the dynamic phenomena after correction, so as to extract infrared spectrum fingerprints of the moving targets and the dynamic phenomena; and (2-4) matching the infrared spectrum fingerprints of the moving targets and the dynamic phenomena with a typical infrared spectrum library to identify the moving targets and the dynamic phenomena; and (3) controlling a two-dimensional servo motor and multi-directional rotation measuring gyroscope and optical fiber gyroscope of the low-orbit satellite to maintain the moving targets and dynamic phenomena within a center of the FOV of the low-orbit satellite.