Abstract: Continuous Phase Modulation (CPM) is an efficient modulation scheme with constant envelop, however its applications have been constrained due to high complexity of the demodulation algorithms. Traditional demodulation algorithms are based on amplitude information rather than phase information,which results in its high complexity. A conception called Phase Distance based on the inherent characteristic of CPM signal is defined and verified based on the 2RC-4CPM. And a novel algorithm used to reduce the path number of MLSE algorithm based on this conception is presented. This algorithm can reduce the computation complexity of the optimum receiver considerably. Simulation results show that the degradation in error performance compared to the optimum receiver is less than 1dB with a proper threshold.