The invention relates to a design method of loading trajectory of a VTL type numerical control skin stretch forming machine, which comprises the following steps: (1) calculating the motion trajectory of an execution jaw according to the geometric configuration of a die; (2) and transforming the motion parameter of a driving mechanism. According to the operating restrictions to the machine tool, the algorithm can verify whether the loading trajectory data of the execution jaw and the transformation data exceed the motion range of the machine tool. The method can previously give out reasonable loading trajectory through theoretical calculation, thereby reducing the number of actual stretches and enhancing the part forming quality.