Abstract: The research on software reliability of digital reactor protection system (RPS) plays an important role to improve the reliability of RPS. Based on analyzing the fault data set collected during the software development of HTR-PM RPS, a new software reliability model involving the fault severity was studied and established. In the practice detecting faults with different severities usually have different levels of difficulty, and the trend of detecting rate with different severities varies with time. In this paper the severity ratio function (SRF) was proposed, and separate models were developed to deal with faults with different severities. This can achieve better prediction and reflect the software reliability in more aspects for engineering application.