Abstract: In order to optimize the efficiency and economy of underground coal gasification(UCG) system in actual production, the efficiency of energy conversion in the process of UCG was optimized.A model test system was used to simulate the UCG under the conditions of 40%,60%,80% and pure oxygen concentration of gasifying agent respectively.Exergy evaluation model of UCG was established by combining the first law of thermodynamics and the second law of thermodynamics.The influence of gasifying agent with different oxygen concentration on exergy efficiency of UCG gasifier was analyzed.The concentration of gasifier oxygen is one of the main factors affecting exergy efficiency of UCG system.Under the condition that the concentration of gasifier oxygen is 40%,60% and 80%,the comprehensive exergy efficiency of UCG gasifier is 67.47%,73% and 78.52%,respectively; the efficiency of external exergy gasifier is 47.36%,61.00% and 57.73%,and the irreversible exergy loss is 32.53%,27.00% and 21.48%,respectively.When pure oxygen is used as gasifying agent, the exergy efficiency of UCG system can reach 84.72%,the exergy efficiency of external exergy can reach 68.86%,the concentration can significantly improve the exergy efficiency of the system and reduce irreversible exergy, and the rate of irreversible exergy can be as low as 15.28%.Irreversible exergy can be reduced by increasing adiabatic combustion temperature, reducing heat loss in gasifier, and increasing heat transfer coefficient.