With the rapid development of optical information processing technology, optical information processing technology based on multimode fiber dispersion is concerned. However, how to exactly excite the required modes has limited the application. There is the analytical solution for graded-index multimode fiber, but not for step-index multimode fiber. Step-index multimode fiber is more suitable for the optical information processing since it has large mode dispersion. The mode excitation of step-index fiber is discussed through Matlab simulation. Simulation results show that different excitation modes can be obtained by changing the conditions of the incident Gaussian beam such as offsetting beam, tilting beam, changing the radius of the beam waist. At last, the mode excitation of tilting beam and the modal dispersion of the fiber are tested through experiments.