The whole performance of an electro-optical imaging system consisting of optical systems and array detectors was explored and optimizing design of the system was discussed. As array detectors were developed and updated in a higher speed, how to improve the optical system to optimize the whole performance of electro-optical imaging system was researched. According to the sampling theory of the sampled electro-optical imaging system, the Modulation Transfer Function(MTF) characteristics of the electro-optical imaging system were studied and the MTF phase average concept for the sampled electro-optical imaging system was discussed. Then, the effect of fabrication and use errors of the electro-optical imaging system on the MTF was analyzed and the calculation formulas of the signal-to-noise ratio of the electro-optical imaging system were given. For a remote sensing electro-optical imaging system, it suggests that the optical system MTF normalized spatial frequency should be set to be 0.5 for matching with the Nyquist frequency of the array detector. By which the system satisfies the demand of the signal-to-noise ratio, the MTF at Nyqust frequency reaches 0.1, and the resolution power is close to Nyquist frequency without the spectrum aliasing.