Velocity obstacle method is more widely used in flight conflict detection and resolution, but it cannot give the time to start resolution, and the range of resolution is different at different time in the approaching process of two aircrafts, and the cost and efficiency of resolution will be affected by the resolution too early or too late. To solve this problem, a method of determining the maneuver time for resolution was proposed based on the velocity obstacle method. Firstly, the velocity obstacle method was used to judge whether there is conflict between unmanned aerial vehicle(UAV) and manned aircraft, adopting the heading resolution strategy, and the resolution range of heading resolution was determined. Secondly, taking the initial detection of conflicts in the surrounding area by the drone as the initial maneuver time for release, and the time corresponding to the minimum heading deviation angle for both clockwise and counterclockwise release of the drone not exceeding the maximum allowable heading adjustment amount as the latest maneuver time for release, an equation was established based on the initial and latest maneuver times to determine the maneuvering range of the drone. Finally, taking the heading adjustment range and the maneuverable range as constraints, a multi-objective resolution model was constructed with the heading resolution adjustment angle and deviation distance as the total objectives, and the Pareto optimal solution was obtained by the enumeration method to determine the heading resolution maneuver time of the UAV, and the trajectory recovery model was established, which enables the aircraft to recover the planned trajectory after completing the resolution. The feasibility and effectiveness of the proposed method in solving the problem of when the UAV starts resolution are verified by MATLAB simulation.