Gottesman, Lo, Lütkenhaus and Preskill (GLLP) have provided the proof of security in Quantum Key Distribution (QKD) with coherent source, which need the estimation of the single-photon gain and error rates. On the other hand, two-way entanglement purification (2-EPP) can reduce the bit and phase errors so as to enhance the secure key rate and communication distance. In former research, the effect of 2-EPP to the average error rates was analyzed and used in the Shor-Preskill proof which is suitable to the cases with ideal single-photon sources. But when it turns to the practical cases with the weak coherent sources in QKD, such as decoy-state BB84, the effect of 2-EPP need to be further studied. Taking Step B as an example, this paper discusses the gain and error rates with 2-EPP of single photons and untagged bits so as to give the precise estimation of the performance of practical quantum key distribution by GLLP proof. The analysis and simulation show that 2-EPP can extend the communication distance even when the sources are not ideal.