The feasibility for the applying of multimode fiber (MMF) in the future long haul,high bit-rate,and large capacity PON is analyzed,and the strengths and weakness of MMF are summarized.Then,initiating from the waveguide theory in the weakly guiding fiber and neglecting the intra-modal dispersion as well as nonlinearity,a simple model for multimode fiber is built,based on which analysis,research and comparison through simulation experiments on some performance improving techniques of MMF are made:offset launching,mode size changing,ring launching and feedback equalization.The simulation results demonstrated that the combination of field shaping (mainly ring launching) and feedback equalization can obviously improve the transmission properties of MMF,because of which,the 10 Gb/s data can travel in the MMF for more than 1 km.