This paper describes the full-automatic solar tracker's principles，structure，main function and application. The full-automatic solar tracker has two tracking mode which can switches freely. The two methods which can switch by themselves achieve high-precision automatic tracking sun all time. The successful development of full-automatic solar tracker in the absence of developed sensors improves the measurement accuracy of direct radiation and the total radiation，Solves the problem which signal wire of direct radiation instrument rounds each other and achieves full-automatic measurement of direct radiation. As full-automatic accurate tracking sun can automatic measure sunshine duration it fills a gap of automatic measurement of sunshine duration in our country. In addition it can carry out automatic measurement of scattering radiation. It has other applications too.