The caliber，focus and other parameter sometimes are ascertaining in the engineering designing of the Electro-Optical system.The method improving the day detecting ability of the system was researched under the restricting situation，Spectrum filtering technology，Bin code and CCD gain can improve the detecting ability of the system，and based on the SNR theory，the limit detecting ability model was established under the three technologies.At last，the models were simulated under the MATLAB soft by selecting proper parameters，the simulation results indicate that the500mm caliber Electro-Optical system detecting ability can improve 2.5 magnitude via adopting the three technologies when the sky background brightness is 10 W·m-2·Sr-1.