A novel method based on motion synthesis is proposed for low-thrust trajectory design and optimization. Firstly, the representation of the low-thrust trajectory was obtained by using the motion synthesis technique. In this representation, the equations of motion was decomposed into three independent motions and the analytical expressions of the three motions were deduced separately. Then the approximate analytical expression of the low-thrust trajectory was obtained by synthesizing of the three motions. Secondly, based on this representation, the original problem was converted into a nonlinear programming problem. Finally, the method was applied to three rendezvous missions: from the Earth to the asteroid 1989ML, to the Mars, and to the Venus respectively. It is shown that the transfer orbits satisfy the mission constraints. Numeral results fully validate the availability and the high efficiency of the method.