Traditionally, well testing has been carried out over a specified time period. The resulting data have been the basis of the analysis of the reservoir. However, more recently, many completed wells have a permanent downhole gauge, from which continuous long-term data are recorded. From this long-term data, we can find that some of reservoir parameters, which were regarded as constants in traditional well testing, might change. In this research, changing permeability and skin factor were considered. With permeability and skin factor changing over time, a new analytical solution was developed. The new analytical solution was verified with numerical simulation results. This solution was applied to real reservoir data recorded from a permanent downhole gauge using an inverse simulation. The results showed that this solution is consistent with real reservoir data.