Fourbar linkage & doublecantilever flat flapgate is a new type of structure on water conservancy project. In order to promote the application and popularize of the sluice, the difficult points on linkage design were studied. Through researching the motion locus & forces acting on fourbar linkage system the mathematical model was developed and used on computers.Thus,the CAD system for solving the design problem of fourbar linkage can be realized.