To establish accurate response model of time delay integration charge coupled device (TDI CCD) image system, the main parts are analyzed in detail, such as CCD sensor, amplifier circuits and video processing circuit, according to the flow of signal processing. The response model for the actual system is established in theory based on the flow processing. The noises of dark current, video processing circuit are tested on a spatial TDI CCD image system. And so is the radiometric calibration under conditions of different parameters and illuminations. The image data of experiments is analyzed. An approximate formula and a precise formula are achieved by linear polynomial and quadratic polynomial of the least square method. The goodness of fit for linear and quadratic polynomial shows that R2 reaches 0.992 and 0.9998, respectively. The radiation character of ground targets can be acquired by the response model. And the choice of reasonable working parameter can also be decided according to the model. The experimental results show that the response model of image system is accurate and the working parameters are reasonable, which meets the demand and improves the efficiency of on-orbit spatial camera.