The PBC00 protocol with excellent security features is studied. For the purpose of preparing polarization states and realizing protocol, the polarization states of the PBC00 protocol are changed to the |1〉, |0〉 and |+〉. For the purpose of analyzing the secutity of protocol, it is assumed that the eavesdropper uses the intercept-resend strategy to eavesdrop during the quantum key distribution (QKD) process. The measurement basic vectors that the eavesdropper may use are analyzed and the key error rates caused by the eavesdropper are calculated. The modified three-state QKD protocol based on the BKM07 protocol is introduced and its security is analyzed. The study results show that the modified PBC00 protocol has a stronger ability to detect eavesdroppers, is easy to implement, and has a practical application potential.