A system of cantilever beam coupled with moving mass is investigated in this paper. Based on the time varying feature of multibody system, a dynamic model of the mass moving along the cantilever beam is established. The time varying motion equation including time varying mass and stiffness is derived by using the generalized Hamiltion’s principle, which results to a partial differential equation with variable coefficients. The nonlinearity of the equation is featured by those variable coefficients. To deal with the nonlinearity, methods of assumed modal shapes, Runge-Kutta are occupied. Then, a time varying system of a cantilever beam with stepwise sections coupled with a moving mass is given as an example. Numerical solutions of the dynamic responses of the system are achieved and certain analyses are made in the form of graphs.