When Global Positioning System (GPS) blended with an Inertial Navigation System (INS), their performances would be remedied both. In this paper, a simulator with an INS module and a GPS receiver module on L1 C/A code signal in Earth-Centered Earth-Fixed frame is presented for testing tight coupling GPS/INS. With the mathematical models formulated to simulate GPS constellation and IMU measurements, both pseudorange and pseudorange rate observables are generated and applied in integrated filter. In addition, an EKF mending scheme is proposed to estimate the error states through expansion around the corrected states other than the preplanned states at each new update. A flight trajectory with multi-maneuver was implemented to test mended EKF-based GPS/INS tight coupling simulator. The results show that the simulator is effective and available.