This paper addresses the synthesis problem of Watt-I six-bar linkage motion generation, and introduces a novel and convenient synthesis approach. The coupler link used as end-effector of Watt-I is connected to a coupler plane of a four-bar linkage, so that the designers need to give the task positions of these two coupler planes. With the method, a four-bar linkage is synthesized firstly, and then a dyad is attached to this four-bar linkage to obtain a six-bar linkage. For four-precision-position problems, four-bar motion generation can produce infinite number of solutions. And for any selected four-bar linkage, we can present an equation of pivot curve that each point on this curve can generate a satisfactory dyad. Based on the planar solution region of four-bar linkage, an infinite number of synthesized six-bar linkage solution can be expressed in a finite spatial region. In addition, the paper still puts forward a circuit and branch defects distinguish method for Watt-I six-bar linkage. The feasible spatial mechanism solution regions without any defects are obtained by using the distinguish method. Application of the spatial solution regions makes designers to be able to understand the information of the synthesized six-bar linkages on the whole, and makes it possible to get the optimal solution that attached more design requirements.