Abstract: QPSK signal with continuous phase solves the issue of envelope fluctuations which are bought by phase jump, but its bandwidth efficiency is lower than conventional 8PSK. To improve the bandwidth efficiency, the continuous phase 8PSK (CP8PSK) is proposed. In this paper, a differential coding rule is used to indicate the 8PSK signal’s phase change in one symbol firstly, secondly 8PSK signal is represented as continuous phase modulation (CPM), and then pulse shaping method is used to shape the phase, which makes the phase continuous, finally, the CP8PSK expression is concluded. In order to reduce the system’s complexity, Laurent decomposition of CP8PSK is deduced. Theoretical analysis proves that CP8PSK signal has constant envelope, and has higher bandwidth efficiency, better power spectrum than QPSK signal with continuous phase. Matlab simulation results also show that CP8PSK signal has constant envelope and has higher bandwidth efficiency, more smooth power spectrum than shaped offset QPSK (SOQPSK) and 8PSK in the same condition, and the probability of symbol error of CP8PSK is little worse than the theoretical value of 8PSK.