To precisely measure the Modulation Transfer Function(MTF) of a space camera at the Nyquist frequency before entering orbit, measuring methods were researched and some experiments for measuring the MTF at the Nyquist frequency were carried out. The schematic diagram of the MTF measurement was given. On the basis of the schematic diagram, the transfer process of a square wave target at Nyquist frequency in whole chain and the contrast ratio model of a terminal target image were investigated. Furthermore, the relationship between the contrast ratio of terminal target image and the MTF at Nyquist frequency and the error formation were given. Finally, the MTF at Nyquist frequency of the camera was measured in a vacuum environment and the obtained results were corrected. The result shows that the MTF of a space camera at Nyquis frequency is 0.213 at a lab condition, and that after the optical system in the camera architecture is 0.602 by conversion. Moreover, the MTF after Time Delay and Integration(TDICCD) is calculated as 0.946, which is close to the theoretical value. These results indicate that the derivation of our method is basically precise, and is capable of satisfying the requirement of space optical cameras for the MTF measurement before entering orbit.