A beam which slides through a prismatic joint is considered as an axial ly moving cantilever beam. The transverse vibration equation is given and the di scretized equation of motion of the beam with timedependent coefficients is de r ived by the mode superposition method. The fixed boundary condition is modified by the adjustment of the cantilever length of the theoretical beam model with th e measured results of its first order natural frequencies under various cantilev er lengths. It is found that the correction lengths to the model beam have no ex plicit relationship with the cantilever lengths. The first order decay coefficie nts are identified by logarithmic decrement method and the decay coefficients of the beam decrease with the increase of the cantilever length. The calculated re sponses by the modified model agree well with the experimental results, which ve rify the effectiveness of the proposed model modification method.