For the problem of real-time route planning of UAV, threats affecting security are analyzed. An improved threat probability model is established. Based on safely avoiding unexpected threat, the impact on flight price of deviating from reservation route is considered so that a new model of flight aiming at shortest total distance is established. In view of single emergent threat and multiple emergent threats in different arrangement forms in the current flight segment, the corresponding optimal route analysis is proposed respectively. Heuristic algorithm based on the short circuit is designed to simulate the examples on three scenarios. Simulation results show that the model and algorithm has validity and feasibility and can have a real-time analysis of emergent threats and plan a real-time relatively safe and economic route.