The mode division multiplexing (MDM) transmission system uses the limited orthogonal modes in few mode fibre (FMF) as the independent channels for information transmission in order to multiply the transmission capacity of the system. FMF uses different modes in the fibre as a new degree of freedom, improving the spectrum efficiency of the system. As FMF has large mode field area, its nonlinear tolerance is high. It not only improves the capacity of optical transmission system, but also avoids the impact of nonlinear effects on the system. Therefore, using the limited, stable modes in the few-mode fibre as independent channels for mode multiplexing can greatly improve the capacity of the system and solve the future bandwidth crisis of single-mode fibres.