Abstract: The supporting stiffness of the floating parts has great effects on the power shared out among planetary gears and on the dynamic load on the meshing gears.A dynamic model of 2K-H planetary gear train has been developed.The factor K_c and factor K_d are introduced for measuring the unbalance of power and dynamic load.For the purpose of reducing the unbalance of power and the dynamic load,a dynamic optimum method is used to determine the supporting stiffness of the floating parts.The dynamic characteristics of planetary transmission designed by optimum method have been analyzed.It is found that the values of K_c and K_d are fairly small.