The importance of period estimation for pulsars in pulsar searching was discussed. In the paper, the radiation signal from pulsars was modeled as second-order cyclostationary process,and corresponding simulation was done to verify its reasonableness. Based on the model, a novel method was proposed to estimate pulsars’ cyclo-period by the coherent statistic of bispectrum, and then one robust strategy for periodicity searches was provided in order to eliminate its sensitivity to data amount. Using the method, the period of PSR J0437-4715 with single pulse was estimated based on original signal, and the period of PSR B1821-24 with double pulses was also estimated based on simulated signal. Results show that, compared with the classical Fourier-transform method, the algorithm is intuitive, effective, of good performance even in case of lower SNR,and also appropriate for the noncontinuous observing data. The method, though with higher computation complexity, can provide a powerful reference for period estimation of weak pulsar signal.