It is common for natural gas to penetrate into annulus of production casing through completion stringing in high pressure sour gas wells. When annulus pressure exceeds the bearing capacity, gas wells will face a great risk of leakage.The location of leakage point is an important reference data for controlling production casing pressure properly and treatment measures. The leakage point can be located by running leakage point detecting instrument, but it will affect the production and with high cost, so it is seldom used presently. Therefore, it is necessary to study the ground pressure relief diagnosis method for locating the leakage point. Based on the principle of multiphase flow, the reason and process of production casing pressure are studied firstly. Three kinds of reasons and characteristics of annular pressure are put forward, and five stages and their characteristics of gas flowing into annulus are also put forward. According to principle of pressure balance after a long time of leaking, a pressure balance model is established, on the basis of which a multi-phase flow calculation method is used to calculate the location of leakage point. By combining annulus pressure relief test and calculation method of leakage point location, the diagnosis method of leakage point location is further formed, and application is carried out to determine the approximate location of leakage point, and effective suggestions are put forward for production management of wells with annulus pressure. The method provides a simple and effective way to determine location of leakage point, and plays an important role in production management and treatment measures.