Abstract: The calculating methods of the thermodynamic properties (TP) of combined cycle power plant (CCPP) wet coal-gas were analyzed which is used in the simulation of gas system. The results showed that wet coal-gas could be treated as ideal gas mixture reasonably and the common equations for the TP of ideal gas mixture were still valid if wet coal-gas pressure was below 732 kPa. The calculating error of the proposed method was small. The wet coal-gas was regarded as the mixture including dry coal-gas and water vapor, then the calculation process and common equations for the TP of wet coal-gas were discussed. The relationship between the fluctuations of the wet coal-gas composition, precipitation of water vapor, temperature fluctuations and the TP of wet coal-gas was analyzed based on the numerical computation. Finally, the handy calculation method of the TP of wet coal-gas was given, which could be used in the simulation of the CCPP coal-gas system under off-design conditions.