Abstract: In order to evaluate the availability of BDS satellites and analyze the influence factors of BDS constellation MEO space backup strategy based on the reliability principle, the influence of BDS satellite fault on the availability of single satellite is analyzed based on the actual situation of BDS satellite. The results show that in the orbit design of BDS MEO backup satellites, the height difference constraint of the backup satellite orbit must consider both the orbital inclination allowable drift and the satellite mean interruption time interval. The effect of orbital maneuver on the availability of single star is smaller than that of single particle inversion (recovery time 12h, 24h), and the combined effect of the three faults is greater than that of unilateral factor.