Combined the screw theory with the principle of a spline curve generation validly, a concept of virtual joints is proposed in this paper. Based on this concept, a novel method of trajectory planning in cartesian space is presented. This method is simple as well as intuitive by virtue of synthesizing the advantages of both the path planning in joint space and that in cartesian space. In addition, through this method a spatial trajectory is obtained easily, which will-go through all the control points and be content with the boundary conditions as well. In order to verify it, an experiment on a 9DOF modular robot platform and corresponding results are given at last.