Point source method can rigorously detect the modulation transfer function (MTF). The method can effectively test the MTF value in all directions and can be used to monitor the on-orbit performance of space detection cameras. However, there is noise in the point source image, and the traditional filtering method is used to denoise it. The detection accuracy may be insufficient, and even the phenomenon that the MTF curve step is approximately 0, which affects the evaluation of image quality. Therefore, depending on the characteristics of the light intensity distribution of the point source image, a point source image-denoising method based on the mean value of the flat area is proposed. This method is quantitatively compared with the traditional filtering method. Simulation and experimental results show that this method increases the detection rate of the MTF curve to more than 90.91%. Compared with the best median filtering in traditional filtering methods, the detection accuracy of MTF can be improved by 98.73%, improving the peak signal-to-noise ratio and structural similarity of images.