The ground forces have to face more and more threat from the space in future operations. The primary threat is coming from reconnaissance satellites. The threat assessment method of optics imaging reconnaissance satellite was studied. The methods to evaluate image quality of remote sensing satellites and the influence factors of image quality were discussed. The ground target discrimination probability model of optics image reconnaissance satellites was formulated, and amended by influence coefficients of illumination, weather and contrast. Taking into account military task, the expressions of threat degree were formulated when target size was given. The threat degree was then divided into several rankings according to the target size to support the military operations decision, and the evaluation method of threat ranking of optics image reconnaissance satellites was presented. Finally, the method was applied to threat evaluation of an optics image reconnaissance satellite, and the result was given.