Considering the problem of too large area the shading disc covered, complex shadow band coefficients and too big diffuse radiation measurement bias, an automatic shading device for diffuse radiometer is designed and realized. Set a shading ball on the automatic sun tracker, drive linkage parallelogram structure with the declination axis arm, the shading ball can rotate synchronously with the apparent motion of the sun and shade beam radiation measured with pyranometer, thus shading beam radiation and measuring diffuse radiation can be realized automatically. The comparison test results show that the automatic shading device can realize diffuse radiation measurement automatically, the accuracy of diffuse radiation readings can get a 18.7% improvement compared with traditional measure system, greatly improves the reliability and accuracy of the diffuse radiation measurement.