In view of the problem that the availability judgment process of traditional GNSS receiver integritymonitoring algorithm is too conservative，and the error protection level value is too large，resulting in pooravailability，the availability judgment process of the traditional algorithm is optimized，the dynamic allocationmethod of the missed detection rate according to difficulty of satellite fault detection，and corresponding calculationmethod about VPL are given. Theoretical analysis shows that the above methods can minimize the error protectionlevel value without reducing the overall integrity index to improve the usability of the receiver integrity monitoringalgorithm. Simulation experiments results show that the usability of the improved GNSS receiver integrity monitoringalgorithm is 1. 5% ～ 2. 5% higher than traditional algorithm.