The aviation users have high requests to integrity of GNSS.Based on the idea of improving the integrity level of GNSS combined system, an algorithm is put forward using protection threshold as an indicator for monitoring the integrity of GNSS combined system for aviation users.The algorithm integrates tightly the different satellite systems with different integrity monitoring methods, which can increase the aviation users' integrity level significantly.The simulation results show that the combined system of GPS/Galileo/Compass can provide global low-altitude flight and polar navigation with security assurance, and the effectiveness and feasibility of the method is proved.