A new approach is proposed to improve BPSK signal parameter estimation performance under low SNR environment. First, the smooth pseudo Wigner-Ville distribution(SPWVD) is applied to the BPSK signal and time-frequency matrix S_SPWVD (t_m,f_n) is obtained, which can estimate carrier frequency of the signal combined with the minimum entropy criterion. What's more, the discontinuity of signal phase is mapped to negative magnitude peaks on its carrier frequency in the time-frequency domain. Then,the code width of the BPSK signal can be estimated by searching the interval between adjacent negative peaks, and then the code rate, code number and phase-coded sequence could also be estimated. Finally, simulations show the validity of the proposed approach.