A new scheme for the forward position analysis of 3-RPS in-parallel manipulator, which is based on the fact that locations of three globular joints form triangle of the moving platform,is presented. In the successive approximation method,the joint positions of globular joints is determined by checking the distances of three sides of moving platform. In general numerical schemes, which determine a solution from a set of nonlinear simultaneous equations, the convergence rate varies with different joint inputs. The convergence of this method depends chiefly on approximative accuracy not on the joint inputs because joint inputs, search region and step are self-modified successively in the process. There is also no influence on the neighborhood of singularity for the solution. The example was given, and running results of the program show the approximative accuracy can be 10-5.