The stochastic master equation approach to molecular motor's directed motion is used and a periodic one-dimensional four-state stochastically unequal interval hopping model is studied. The drift velocity V, the diffusion constant D and the randomness parameter r of the steady state are all be obtained. By comparing the simulated curve with experiment about drift velocity V, diffusion constant D and randomness parameter r versus ［ATP］ and external load F, the kinetic behaviors of a molecular motor under external load F are qualitatively analyzed.