Continuity and availability are important indicators of GNSS performance assessment. In this paper, uniform assessment criteria, models and algorithms are used to evaluate and compare the continuity and availability of GPS, GLONASS, Galileo, and BDS. A total of two years of data from 2015-2016 were used in the study. The results show that the average continuity of all satellites in GPS is 0.9996, and there is still a small gap between the requirements of GPS SPS (0.9998). The average availability of all satellites in GPS is 0.9903, which is better than the 0.957 proposed by GPS SPS. The continuity and availability of GLOANSS systems are 0.9999 and 0.9911, respectively, which are better than those of GPS SPS. The average continuity of all Galileo satellites was 0.9993, which did not reach the standard of GPS SPS PS. This may be due to the fact that the Galileo system newly launched satellites are still in orbit test. The average availability of 0.9990 is superior to that of GPS SPS. For BDS satellite navigation systems, the average continuity and availability of all satellites are 0.9979 and 0.9875, respectively, although both meet the requirements of the BDS standard service, but their continuity is much lower than that of GPS SPS. Continuity is the performance that BDS urgently needs to improve in system construction.