Quantum state in the channel is affected by several factors, which will bring detection error and make secure threshold of quantum distribution protocol unable to judge whether eavesdropping exists. We analyze the factors in real quantum channel, which affects the quantum state, develop a channel model, and derive a judging eavesdrop secure threshold formula of BB84 protocol based on an imperfect channel. Through calculating and simulating, the formula is proved to be more accurate to estimate the secure threshold of the BB84 protocol and more effective to judge eavesdropping. Meanwhile, this method can improve the security and the efficiency of key distribution.