In order to realize the highly accurate observation of solar irradiance, an automatic sun-tracking system was designed by using solar motion trajectory tracking and four quadrant detector tracking. The working principle of four quadrant detector was analyzed and the uniformity were tested. The deviation of the quadrant detector was measured. The hardware, software and structure design of the system were accomplished. The system using for data acquisition, signal processing and feedback control as C8051F064 was realized. The tracking precision of the system was tested in the laboratory and the results showed that the tracking deviation was less than 0.1°. The system ran successfully in practice. The scheme of system was proved to be stable and highly precise.