On-line Autonomous Route Planning is the prerequisite for Unmanned Aerial Vehicles (UAVs) to perform area search mission effectively. Firstly, to satisfy the real-time requirement of UAV search path planning, an autonomous route planning framework which integrates fixed search mode with dynamic search mode is presented. Then, based on basic Rapidly-exploring Random Tree (RRT), a modified RRT-based UAV route planning algorithm is proposed. The modified RRT method has introduced heuristic information, and improved the extension of RRT node, which can reduce the node number in a search tree and time cost. Simulation results demonstrate that the method is real-time and effective.