Aimed at the problem that binary phase shift keying (BPSK) has 180°phase jump phenomenon, and the traditional definition of instantaneous bandwidth can't reflect the frequency properties of the BPSK. This paper defines the instantaneous bandwidth of BPSK as the second order central moments of time frequency distribution, derives the instantaneous bandwidth of the expression, and obtains the mutation peak value of the instantaneous bandwidth of the phase transition point to the actual interception BPSK generated envelope gap phenomenon. The paper utilizes the envelope gap for realizing the identification of coding mode of BPSK by using threshold detection, and analyzes the influence of the signal related parameters on the depth of envelope gap. The simulation results verify the correctness of the theoretical analysis.