It is used attenuated laser pulses as the signal source rather than single photons in practical QKD system. The channels is used to transmit are lossy. Eve's eavesdropping ability is limited. On the basis of above three points,a eavesdropping device is presented. Meanwhile,the study is analyzed the calculation of the effective average Alice/Eve mutual information and Alice/Bob error rate's upper limit in six states QKD protocol. It shows that Breidbart eavesdropping/beamsplitter is the more effective one compared with Breidbart eavesdropping(B/B strategy).