According to the fact that the fourth-order cumulant of the Gaussian White Noise is equal to zero,the fourth-order cumulant of the DSSS/QPSK signal is studied and the time domain and frequency domain detection methods based on the fourth-order cumulant different slices c_4x(0,0,0)、c_4x(τ,τ,τ)、c_4x(0,τ,τ) and c_4x(0,0,τ) are proposed.Experimental results show that the detection performance of the method in frequency domain is better than that of the method in time domain.Trading off the detection performance and realization difficulty,the frequency domain detection method based on c_4x(0,0,τ) is the best.When the false alarm probability is set to 1%,its detection probability reaches above 50% at SNR -11dB.The proposed methods don’t involve extensive matrix operations,their computation complexities are low,and they are easy to implement.