Abstract: This study proposes a modified A* algorithm to solve the problem of realtime unmanned air vehicle(UAV) path planning in a large 3D battlefield environment. Since the UAV has to meet the requirements of high survival rate and low fuel consumption in low-altitude penetration，the algorithm took the flight altitude，detected probability and flight length into consideration to search the optimal flight path between two waypoints. Meanwhile，to satisfy the UAV perfor-mance constraints，such as safety altitude，rate of climb，and radius of turn，the author suggested a series of optimization algorithms to get the final flyable path.Experimental results show that these algorithms have good convergence and high efficiency andprovided a optimal trajectory for decision-makers.