The detection ability of an electro-optic system is related to sky luminance, the electro-optic (EO) system and detector. The primary specification for expressing detection ability is signal-to-noise ratio. Based on analysis of the effect factors, a calculation formula of detection ability, including the imaging number, is derived. The influence of the sky luminance, parameters of the EO system, and the dispersion of the object on the detection ability is mainly analyzed. Simulation is conducted through an example according to the space object model, and the theoretical results show that the space-object can be detected when elevation is greater than 30°. Theoretical results fit with the experimental research. The effects of sky luminance, parameters of system, dispersion of imaging point on detection ability under different conditions are quantitatively learned. Moreover, the scientific basis for the optimization design of the system and development of practical work is provided.