Shape-based approximation is an effective method for the low-thrust trajectory design. However, the vast majority of methods assume that the motion trajectory is a specific shape, and there is no constraint on the thrust acceleration. In this representation, according to the issue of low-thrust spacecraft rendezvous, a new method for transfering trajectory design was proposed. Based on the polar coordinates, the trajectory design was successfully turned into solving polynomial coefficients problem through three degrees of freedom motion model built by introduced polynomial function. Meanwhile, the dynamic characteristics of the trajectory as well as the constraint equations were deduced. Subsequently, the optimal polynomial coefficients were solved by the approach of sequential quadratic programming. This method can not only solve problems with a greater number of free parameters, but also meet the thrust acceleration constraints. The simulation has confirmed that this method is accuracy and availability. It can provide a certain technical reference for the trajectory design and fuel consumption estimation during the preliminary stage.