A diagnosing method to solve pressure and temperature with the predicted values of equivalent potential temperature, total water content and density is developed for the NTU-Purdue Nonhydrostatic Model. The efficiency and accuracy of the method is examined for a wide range of atmospheric conditions. We found that very few iteration steps are required because of the fast convergent rate of the method. The diagnosing method is tested on moist mountain waves, and the results are consistent with our present understanding of the phenomenon. The model seems to be extremely stable, as it endures very long integration without showing sings of deterioration that is common in a numerical solution. It is also very accurate in depicting many detailed structures of the mountain waves.