The present invention discloses a symmetrical extended binary phase shift keying modulation and demodulation method. By taking an extended binary phase shift keying modulation mode as the basis, the symmetrical extended binary phase shift keying modulation and demodulation method comprises a symmetrical extended binary phase shift keying (S-EBPSK) modulator and an S-EBPSK signal demodulator, the S-EBPSK modulator corrects a modulation waveform corresponding to a code element 0 relative to the EBPSK modulation, so that the code element 0 generates the phase jump symmetrical with a code element 1 at the (N/2+1)-th carrier cycle. A modulation demodulation system of the present invention has the anti-noise and anti-fading performances obviously better than an EBPSK system, overcomes the disadvantage in the EBPSK that the threshold decision can not adapt to the signal amplitude change, and saves the modules, such as an automatic gain control module, an adaptive threshold decision module, etc., thereby simplifying the structure of a receiver substantially, and having the equivalent transmission rate and frequency spectrum utilization rate with the EBPSK.