The interoperability of global navigation satellite systems (GNSSs) has a significant impact on the service performance of GNSSs. To evaluate the GNSS interoperability, this paper creates an evaluation algorithm with such modules as space section, user section and environment section. The proposed algorithm evaluates the service performance with several common parameters, namely, the dilution of precision (DOP), navigation satellite system precision (NSSP), navigation satellite system integrity (NSSI), availability and continuity. The parameters of the algorithm could be configured based on the existing GNSS data or our self-developed data. Then, the proposed algorithm was applied to evaluate the service performance of three difference scenarios: Compass-only, Compass + GPS and Compass + GPS + Galileo. The results show that the combination of different GNSSs can greatly improve the service performance of GNSSs in both time and space.