In order to meet the much smaller envelops of parallel manipulators than those of serial ones and to overcome the disadvantage of easily going beyond their envelops while trajectory planning, a method to plan a linear trajectory with parabolas blends has been provided in detail in this paper, which can ensure the continuity of velocity at the two end of a stroke. Finally, the validity of this method has been verified through a planning example of 3 degree of freedom parallel robot.