Industry experience has shown that the most serious problems resulting from sustained casing pressure (SCP) are caused by a tubing leak. In addition, the causes of SCP in the “A” annulus are unclear and there is still no effective detection technology. In order to understand the mechanism of SCP in the “A” annulus of an offshore gas well, a ground-based diagnostic test technology of SCP in the production casing annulus has been investigated. 54 kinds of gas leakage paths leading to SCP are clarified using Fault Tree Analysis (FTA). We identified that a tubing leak is the main reason for SCP. Hence, a model is established for leak detecting and locating for a gas well, based on pressure distribution in the wellbore. And combining a detection index including pressure, flow, temperature, annular liquid level and gas components, a diagnostic test system is developed to monitor well data on offshore platforms. On the base of the model for leak detecting and locating and the testing data, two cases of SCP resulting from production tubing leakage are determined by a graphical method. A tubing leak causes SCP if the leakage point is above the original balance point (the intersection of the profiles of tubing pressure and production casing annulus pressure upon well completion). And the wellhead pressure increases along with the falling of the annular liquid level. If the leakage point is below the original balance point, the annular liquid level falls and the pressure at wellhead remains zero.. At the same time, a quantitative relation is developed of the wellhead pressure increases along with the falling of annular liquid level as a result of a series of pressure bleeding-down operations. It is of significance to improve the safeguard technology level of oil and gas activities.