This paper establishes a geometric model of multi-band mosaic imaging from the same orbit by agile satellites, and introduces a self-write simulation software. Geometric parameters of each band are calculated based on the attitude control ability of the satellite and the mission requirements. Considering the different ground resolution and the imaging angle of each band, two new concepts, Gradient Entropy and Structure Similarity Parameter are presented. These two values are used to evaluate the change of image quality caused by agility, and help to estimate the effect of the mission. By building the geometric model and calculating the agile information with the program, we propose a new approach of forward analysis of agile imaging, which helps users evaluate the image degradation.