The rapid development of urbanization presents new requirements to autonomous collaboration capability of UAVs both in military and civilian application.Considering that the current mission planning algorithms for UAVs have the drawbacks of high complexity and poor real-time performance, we proposed a novel UAV mission planning method based on urban environment.Simple mathematical models were created for the urban buildings at first.Then, the improved visibility graph was applied to create all the shortest paths based on the models, and knapsack problem algorithm was used to allocate the missions.Simulations show that the algorithm can make an accurate planning that satisfies the real-time requirements.