Abstract: A method to establish the kinetic differential equation of plane mechanisms with multi-degree of freedom was studied.On the basis of Lagrang eequation the generalized coordinates whose number was equal to the number of thedegrees of freedom of the mechanism were introduced,and according to the variation characteristics of the kinetic energy and instant power of the mechanism the explicit formulas of the equivalent moments of inertia and the equivalent forces were set up.In the end the kinetic differential equations of plane mechanisms with multi-degree of freedom was deduced.