Guiding systems serving modern astronomical telescopes are usually subjected to atmospheric and wind-borne disturbances that result in inaccurate calculation of the center of gravity of the guiding beacon. In order to solve this problem effectively, the sub-pixel real-time gray projection algorithm is nested into the algorithm of center of gravity of auto guiding system, which reduces the jitter of the center of gravity in a closed-loop cycle of auto guiding system without losing time resolution and achieves the goal of improving the performance of auto guiding system. First of all, in the paper, we analyze that to implement high performance auto guiding system, obtaining high real-time and small error guiding beacon's center of gravity is significant, and point out that gray projection algorithm plays an important role in the course of obtaining the guiding beacon's center of gravity. Furthermore, after analyzing the reason that classic gray projection algorithm is able to be combined with the center of gravity to increase the performance of auto guiding system, we modify the classic gray projection algorithm in the speed and accuracy so as to combine the modified algorithm with the algorithm of center of gravity of auto guiding system and achieve the goal of improving the performance of auto guiding system. Finally, we test our auto guiding system with the algorithms mentioned above in a 400 mm aperture telescope, and conclude that our system can obviously decrease the random jitter caused by wind load at the cost of less decreasing temporal resolution, and achieve the goal of improving its performance.