The scanning pentaprism system has advantage of simple structure, quick optical testing and providing precise measurement of low order aberrations, it is an effective method for guiding optical manufacturing of large aperture flat mirror. In order to improve the scanning pentaprism technology, this letter makes detailed analysis on the error sources include pentaprism manufacturing error, thermal effect, components′ position motion, mapping error, autocollimator measurement uncertainty in the scanning pentaprism system, and derives their exact influence on the system. The analysis results show that the system′s measurement uncertainty on single testing point can be about 230 nrad under current laboratory environment, and the accuracy of scanning pentaprism system is mainly limited by the measurement uncertainty of the autocollimator and the thermal effect, which providing the proceedings that should be cared for improving the system performance. This study also gives an important reference for the error analysis and precision distribution in design of the scanning pentaprism system.