According to the folding wing morphing aircraft, designed a planetary gear train of folding mechanism, which suit assembly in the small space of morphing aircraft. On the basis of the systematic study of the planetary gear design goals, variables and constraints, describes the floating-point coded genetic algorithm, using the MATLAB optimization toolbox to optimize its main parameters. Research shows that the structure weight loss 16.7% compared with the safety coefficient method, achieve the minimum weight of the folding mechanism in the case of meeting the design specifications. Therefore the floating-point coded genetic algorithm optimization solution for planetary gear mechanism design is an effective optimal design method.