In order to plan trajectory of unmanned aerial vehicles (UAV) in three dimensions, an algorithm based on geometric method is advanced. The path of UAV is analyzed, which could be composed by a set of lines, arcs and helixes. The relationship between path feasibility and the max over loading coefficient, velocity and height limitation of UAV is researched. And the restrict condition of optimal path is founded. The geometric method steps of programming UAV path are discussed. The path is designed by geometric method, in which UAVs flexibility is embodiment. The formulation results exhibit that paths designed by geometric method are well applicability.