In order to meet the highprecision requirement of autocollimating system and expand its application scope, the error sources in large aperture thin beam autocollimating system were analyzed based on the principle of autocollimating in detail. The errors introduced by the incident beam divergence and CCDs defocus, the error introduced by the optical element states, the mirror flatness error, the aiming error and the principle error were quantitatively analyzed. And also the error introduced by external environment was analyzed qualitatively. Through the quantitative and qualitative analysis, the influence of each error source on the system precision is presented, which will provide important theoretical support for system design.