A PNT service system in the form of a hosted payload piggybacked on the common platform of LEO constellation is presented as a complement, backup and verification for GNSS system. The development status of LEO-based PNT is summarized, the strategy and overall framework of this LEO-based PNT system are presented, then the working principle and service performance of this system for backup and verification for GNSS are analyzed. Finally the key technologies for system construction are introduced, and the feasibilities of strategy and technologies for the PNT system are preliminarily demonstrated.