Synthetic aperture ladar (SAL) has the characteristics of long imaging distance, high resolution and fast speed, and plays an important role in the field of space-borne imaging. Aiming at the problems of weak return signal, high noise level and poor imaging quality in space-borne SAL imaging, an idea of continuous long-term observation near the intersection point is presented. A mathematical model of space-borne SAL imaging theory is established by using optical heterodyne detection, and the return signal equation and imaging signal-to-noise ratio (SNR) are obtained. The processing flow, image resolution and mathematical simulation of space-borne imaging with different SNR are given. Theoretical analysis and simulation show that when the SNR is high, any sub-data can form a high-resolution image. When the SNR is low, the target sub-image is formed by continuous long-term observation, and the method of combining all sub-images enhances the SNR of the target image and improves the image quality.