In order to systematically study the thermodynamic performance of low concentration gas power generation and waste heat recovery technology in coal mines and improve energy recovery efficiency, through the study of "coal mine gas extraction cascade power generation system", the thermodynamic equations of gas internal combustion engine, waste heat boiler, back pressure steam turbine, ORC generator set and other ancillary equipments were established respectively by theoretical analysis method. Then an integrated model which contains all equipments equations has been accomplished, through sequential calculation and iterative calculation, the final calculation results of "coal mine gas extraction cascade power generation system" were given. The results show that 22.3% of the chemical energy contained in gas is directly converted from gas internal combustion engine to electric energy. The waste heat utilization system can recover 2.42% of the electric energy. The total thermal efficiency of power generation reaches 24.72% and increases by 10.85%. However, the overall thermal efficiency of gas power generation system is still low and needs to be further improved. The theoretical calculation model established in this paper can clearly show the energy utilization efficiency level of the existing gas internal combustion power generation system and the degree of energy recovery of the waste heat utilization system. This model is a complete index evaluation model for the gas power generation and its waste heat utilization system, and has a strong guiding significance for the optimization and perfection of the gas power generation and waste heat recovery system.