  We present a method without decoy state to estimate faithfully and
efficiently the yield and quantum bit error rate of single photon pulse for
BB84 protocol. The numerical calculations show that key generation rate and
maximal secure communication distance of our method are better than results of
the decoy state method. As we know, it is the first time we present this method
and the idea is very novel. Furthermore, our method can overcome the drawbacks
of decoy state method. In our method, the maximal secure distance is
insensitive to the deviation of dark count rate from exact value.
