A research has been carried out on the influence of six four-mode combinations of seven fiber linear- polarization (LP) modes on the mode division multiplexing system in the multi-mode fiber with a core diameter of 50 μm. The seven-LP modes are composed of three reported low-order modes (LP01, LP11, LP21) and four modes (LP02, LP12, LP13, LP14) with significant differences in propagation constants (β). Six different four-mode combinations are selected according to the pattern spot shape and mode coupling characteristics, followed by a simulation test on the Optisystem platform. The results show that with short distance transmission performed in modular division multiplexing, the mode combination of LP01, LP11b, LP12a and LP21b is characterized with the highest Q factor and the optimal eye diagram effect, indicating that this mode combination achieves the best transmission effect.