Abstract: Objectives: To improve the leakage envelope phenomenon of BDS-3 SISA_oc integrity parameter to the comprehensive ranging errors induced by the orbit radial and clock errors, an improved SISA parameter index conversion algorithm based on 395 days of historical data is proposed. Methods: The algorithm consists of two parts: SISA_ocb (SISA parameter associated with orbit radial and clock accuracy) and SISA_oc1 (SISA parameter associated with satellite clock speed accuracy) algorithm optimization. In SISA_ocb value optimization, the RMS maximum of orbits radial and clock comprehensive error of all satellites in history is taken as the minimum limit of SISA_ocb. In the optimization of SISA_oc1 algorithm, the maximum of first-order coefficient extracted through clock error fitting is compared with SISA_oc1 calculated by free value formula of independent variable to determine the constant N in the SISA_oc1 parameter calculation formula. Results: By analyzing measured data of 8 months, the advantages of the proposed algorithm are verified. The new algorithm can achieve the “zero” leakage envelope of SISA_oc parameter for satellite radial and clock errors (excluding significant anomalies in broadcast ephemeris for satellite abroad), and reduced the leakage envelope event of BDS-3 integrity parameter caused by unreasonable SISA broadcast values. Conclusions: The effective optimization algorithm and analysis results for BDS- 3 SISA_oc parameter in this paper can provide a valuable reference for the upgrading and construction of BDS-3 system integrity service.