The principle and the expression of accuracy reliability of some kinds of sun sensor were analyzed to find that the accuracy of the compound sun sensor based on the existing device have the potential to be enhanced. Based on the device used in disquisition, it was confidence that in theory the absolute accuracy reliability of angle resolution can be at digitally 10-5, and the accuracy can be improved digitally 2; along with the progressing of the device, the accuracy of sun sensor to be used in future can reach digitally 5, or even more.