Existing studies on the secret key generation based on physical layer wireless fading channel are largely open under active attacks. In this paper, we discuss how the man-in-the- middle (MITM) attacker, who acts as a transparent relay, attacks the training phase in wireless communication to improve the information leaked rate. We prove that the MITM attacker can intercept all of the channel information when all of the paths pass through the attacker. We present the closed-form solution of information leakage rate and achieve the optimal attack effect when the transmitter can directly communicate with receiver besides the paths by way of attacker. The results show that the MITM attack can cause new challenges for physical-layer security since it leads to higher information leakage rate. Numerical results are presented to further demonstrate the potent threat of MITM attack.