The average formation pressure is a significant indicator of reservoir development and the foundation for reservoir capacity analysis and dynamic evaluation， which reflects the dynamic aspects of the reservoir development process. However， the conventional shut-in method of measuring the average formation pressure affects production and is inefficient. Therefore， how to obtain the average formation pressure changing with the reservoir production conveniently and quickly is of great significance for the whole reservoir. The conventional method of calculating the average formation pressure does not consider the water influx because the reservoir bottoms with both gas cap and bottom water are connected to the strong water body. Thus， this method is no longer suitable for the reservoirs with gas cap and bottom water. Considering the characteristics of the reservoirs with gas cap and bottom water，this study proposes a new water influx calculation method，namely the fitting deficit volume method to predict water intrusion， which is combined with the material balance method and the function of average formation pressure is established. By solving this function， the average formation pressure in the production process can be obtained. The comparison with the numerical simulation results shows that the relative deviation between the average formation pressure calculated by this method and the numerical simulation is small，only 0.054%. The findings demonstrate that the method does not require a shut-in of the wells， has no effects on production，and just calls for readily available dynamic and static information. It is simple to use and has a high degree of accuracy. It can be concluded that the method can obtain the average formation pressure economically and efficiently. Based on the average formation pressure， the drive index of the reservoir and the phase recovery can be calculated， which provides an important basis for the subsequent development of the reservoir.