This paper presents an improved algorithm for classification of M-Differential Phase-Shift Keying(MDPSK) signals based on eumulant. The feature proposed in the algorithm is invariant with respect to constellation scale, rotation, the shift and the carrier frequency offset between transmitter and receiver.The invariant property is proved theoretically.Through computer simulation the performance is evaluated and the results show that the improved classification algorithm is better and valuable in practice. This paper presents an improved algorithm for classification of M-Differential Phase-Shift Keying (MDPSK) signals based on eumulant. The feature proposed in the algorithm is invariant with respect to constellation scale, rotation, the shift and the carrier frequency offset between transmitter and receiver. The invariant property is proven theoretically. Through computer simulation the performance is evaluated and the results show that the improved classification algorithm is better and valuable in practice.