A lot of reinforced concrete (RC) frames have being collapsed during Si Chuan Earthquake (SCE) in western China May 12th.2008, at the same time others have being injured on several levers. In recent years how to evaluate reliability and fragility of the buildings and search reasonable design practice of seismic strengthening of these buildings is urgent mission. One goal of fragility analysis is set up relation between vulnerability and seismic intensity. A set of stochastic earthquake waves compatible with the response spectrum of China seismic code selected to represent the variability in ground motion. Dynamic inelastic time history analysis was used to analyze the random sample of structures. In the end structural weak position also be pointed as valuable consultation for diagnose these buildings and fragility curves of typical middle-storey RC frames of China was obtained finally.