In recent years, many earthquakes occurred in our country. Some is big, but most are small or middle earthquakes. Usually earthquake damage ratio of buildings in low intensity area can be attained from on site survey. But it is important that how to evaluate the seismic performance of existing building stock subjected to high intensity in earthquake-affected area. In this paper, the expectation and variance value of damage probability in known intensity is calculated by making the probability disperse. Supposing that the damage probability in each intensity obeys to Beta distribution. The expectation and variance value of damage probability in other intensity is given by comparing to that of criterion earthquake damage matrix. So the parameter of Beta-distribution function is calculated from the expectation and variance in other intensity. Then the probability of each damage grade in other intensity is gained. This approach makes the damage matrix which is suitable for this region complete and provides the reference for earthquake damage prediction and earthquake economic loss evaluation. The result showed that this approach is feasible.