Most of the telescope focusing systems adopt the measuring distance method to focus the quick-moving target because the imaging position of moving target is constantly changing. The focusing system calculates the focal position, controls the motor according to the distance of the target. This focusing method has a faster focusing and a better real-time performance compared to the image focusing method based on the image quality. But restricted by the external environment, the precision of instruments and technical level, Distance measuring focus system(DMFS) generally have low precision, higher dynamic adjusting delay problem. This paper mainly analyses the main error sources affecting the accuracy of DMFS, aiming at the existing defects of commonly used current speed compensation method, put forward a kind of solution path delay lag method predicted method measuring focusing system, and then simulate it, the result shows that this method can greatly improve the precision of DMFS.