The CCD Auto-Guiding Star System is a significant part in the telescope control system for minimizing the tracking errors, and acquiring high-resolution data. In this paper, the improved algorithms of the off-axis CCD Auto-Guiding Star System is designed and applied. Especially the de-rotator algorithm is added for the large-field Alt-Az Telescope to keep stars at original positions. The software that can communicate with the CCD camera is designed to collect, analyze and process data, and then sent data to Alt-Az control system in real time less than 160ms. The algorithms to calculate centriod of stars are novel. The experimental results were high-resolution and reliable for system, the RMS of star offset vector can arrive 0.03 pixel. In addition, experiments showed that the software could work steadily in long time. Now the CCD Auto-guiding software has been applied on the f/8 reflecting Alt-Az telescope with 2.5m aperture, 1-degree field of view.