Time-delay estimation is widely used in sonar and radar applications. In order to estimate time delay accurately in variable signal to noise ratio(SNR), an improved algorithm of adaptive time delay estimation(TDE) based on fourth-order cumulants has been proposed in this paper. The single adaptive filter in the conventional model is separated into two adaptive subunits connected in cascade, one is to track the delay, the other to track the SNR, so as to obtain the accurate signal time delay estimation in the low and variable SNR. The theoretical analysis and simulation results confirm the validity and feasibility of the proposed algorithm.