Using the renewal model to compute the conditional probability that an earthquake occurs in the next few decades， the time that it has not occurred in the last T
years is needed. However， due to lack of records of historical large earthquakes for many faults， the Poisson model may underestimate the possibility of the occurrence of major earthquakes. As for the active faults where the historical strong earthquake records are absent， this paper proposed a new method for calculating the conditional probability by using the length of recorded quiet time as a parameter (Ts
). And then the Tazang fault segment was taken for a case study based on the new method. The result shows that the occurrence probability of large earthquakes is about 0.0649 in the next 50 years along the Tazang fault segment.