The dominant limitations for the transmission performance of mode grou p diversity multiplexing (MGDM) systems are inter-symbol interference (ISI) and inter-channel interference (IC I) existing in transmission link.And typically,complex equalization algorithm will be employed to compensate these i mpairments at the receiver. However,the signal processing will increase the computational complexity and th e difficulty of the implementation of hardware.In this paper,we investigate the relationship between different incide nt conditions of the signal light and the number of the excited mode groups,and propose the method of optimizing the inci dent conditions of the signal light at the transmitter,in order to reduce the ISI and ICI existing in tr ansmission link to the minimal level and improve the transmission performance of the system to a great extent.By combining the t heoretical calculation and simulated verification,the optimal conditions of the incident signal light are obtained,a nd the simulation results show that after the optimization for the core-offset distance and the Gaussian optical spot siz e at the transmitter,the MGDM system can realize that two 10Gbit/s signals are transmitted over 400m multi-mode fiber without any electronic dispersion co mpensation and the bite error rate is lower than 10－3.