A multimode interference basic theory of singlemode-mutlimode-singlemode (SMS) is described. Numerical simulation is performed to multimode fiber with graded-index and step-index using finite difference-beam propagation method (FD-BPM). Features such as self-focusing location and exciting mode are compared to show the advantages of multimode fiber with graded-index. According to self-focusing effect, focusing and diverging functions of multimode fiber with graded-index are analyzed on theory. The shorter fiber is used in lens and the main applications at present are summarized.