At present， sustained casing pressure （SCP） occurs frequently in the development of high temperature and high pressure gas wells， indicating that leakage happens along the well， and the well integrity is damaged. The basic cause for sustained casing pressure is failure of the well barrier， which leads to abnormal annular pressure in gas wells and hence threatens safe production of gas wells. It makes sense to analyze the annular pressure build up test data to evaluate the leakage rate of cement sheaths and control the risk of SCP. Five SCP modes were analyzed， with the mathematic model established for gas infiltration flow in the two cases where both cement returns and doesn’t returns to surface. The cement integrity was evaluated using the annulus pressure diagnosis test data， which could not only predict the trend of the annulus pressure， but also obtain the equivalent leakage rate of the cement sheaths. The results of this paper can be used to conduct the cementing design and provide references for management and control of sustained casing pressure.