According to the seismic data from 1911 to 1994, division of seismic periods, seismicity features and the trend of strong earthquake in Tibet have been discussed in detail. Time distribution of strong earthquakes and the probability of occurrence have also been analysed by using statistical method. The results show that the region has gone through three strong-earthquake cycles since this century,and now it is between the end of the quiet episode and the beginning of the active episode in the 4th cycle. There will be several swarms here in the future. During 1995 and 1996, the occurrence probability of M_S ≥ 6.0 earthquake is 80%-87%, and that of M_S ≥ 6.5 earthquake is 60%-76%in Tibet region.