GMSK modulation signal is constant envelope with high spectrum efficiency, but its waveform storage and orthogonal modulation is too complex due to phase path computation and memory address mapping. Joining the separate Gaussian filter into MSK modulation circuit, a simplified design of GMSK modulation is proposed, which has the advantage of simple structure and Gaussian filter design flexibility. The time-domain and frequency-domain simulation tests the validity of simplified GMSK modulation design.