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According to the present problems that the forward displacement analysis of most platform Stewart parallel mechanisms with six links can not be described with whole analytical solutions, and a very few of them can be described with whole analytical solutions and also with these difficulties, including calculation and programming, four kinds of platform Stewart derivative configurations with six links were designed, and a semi-analytic algorithm which combined with numerical method and analytical method was established. By adding six virtual links, the four derivative configurations can be reconstructed into the same kind of 12-6 platform topological configuration. Compatibility equations of reconstructed configuration were derived, and aimed at four kinds of derivative configurations, the numerical solution of the length of the virtual links was derived. Based on the topological relations between feature points on the moving platform, the whole analytical solution of the forward displacement analysis of the reconstructed configuration was derived. Furthermore, the accuracy, efficiency and stability of the semi-analytic algorithm and the traditional numerical method were compared and analyzed respectively. Numerical examples showed that the accuracy and stability of the semianalytic algorithm was at least two times of that of the traditional numerical method, but the efficiency of the traditional numerical method was seven times of that of the semi-analytic algorithm. Meanwhile, three selection principles of configurations were obtained, which established theoretical foundation for the engineering application of six links parallel mechanism.