The hover duration performances of miniature VTOL(Vertical Takeoff and Landing) air vehicles depend on the performance of propulsion system. Based on the mass models of battery, motor and ESC(Electronic Speed Controller), and the relationship between provided force and required power of propeller, the duration calculation model of vertical takeoff and landing air vehicles was established by using the battery constant-current discharge model. The influence of propulsion parameters and payload on vehicle duration is analyzed, which would be used for preliminary design of VTOL air vehicles and components selection of propulsion system.