After investigating the characteristic of variable parameter compound chaotic system, the method to produce compound chaotic system with variable parameter was introduced and compound chaotic system with more Lyapunov exponent to chaotic system with parameter was produced. Newton optimization iteration methods that based on variable parameter compound chaotic system was combined with mechanism synthesis theory, and then the solving method of mechanism synthesis problem based on variable parameter compound chaotic system was put forward. As an example, the problem of function generation for planar four-link mechanism was considered for synthesis and approximate synthesis at given positions. This made multi-projects selecting possible. The method is adaptive to planar multi-linkage and special mechanism.