Space cameras are the major devices used for space situational awareness and their imaging quality therefore needs to be monitored in real time. However, rotating satellite platforms to observe ground targets to measure the modulation transfer function (MTF) increase the technical risk and cannot perform multiple measurements. Thus, research is required for developing a method without platform flipping for real-time measurement of the MTF of the space camera on-orbit. The final point-based point MTF detection method utilizes the common star points in the deep sky background as the target of the on-orbit MTF measurement, combined with the point source method to study its on-orbit MTF measurement method, and utilizes Gaussian fitting positioning and multi-graph registration to reconstruct the point spread function. Simulation and experimental results show that the proposed method has desirable noise immunity. When the standard deviation of random noise is 40, the mean square error of the MTF is in the order of 10 ^-4, and the detection effect of small star targets is good. When the mean square error of MTF is less than 0.2 pixel, the smallest radius of the circle of confusion that can be detected is 0.9 pixel.