Abstract: A new approach of dimensional synthesis is introduced for Watt-I six-bar rigid-body guidance mechanisms. Through establishing the mechanism solution-region,this approach can get all feasible solutions for the finite separated four-position problem of the rigid-body in the end. This paper firstly gives the solution-curve formula and recommends the methods of solution-curve mapping and forming plane solution-region. After that,a circle and branch defects decision is proposed for this type of mechanism. Through this defect decision,the feasible region of no-defect mechanism solutions is acquired,which makes a designer be able to get much better mechanisms that meet much more design requirements,avoid the blindness of selecting mechanisms and improve the design efficiency. Finally,the synthesis process of the approach presented in this paper is illustrated through a synthesis example,which proves the practicability and effectiveness of this approach.