In the current study, simulations based on the engineering equation solver(EES) software are performed to determine the suitable working fluid for the simple organic Rankine cycle system in different temperature ranges. Under the condition of various temperatures and a constant thermal power of the flue gas, the influence of different organic working fluids on the efficiency of the subcritical organic Rankine cycle power generation system is studied, and its efficiency and other parameters are compared with those of the regenerator system. It is shown that the efficiency of the subcritical organic Rankine cycle system is the best when the parameters of the working fluid in the expander inlet are in the saturation state. And for the organic Rankine cycle, the R245 fa is better than other working fluids and the efficiency of the system reaches up to 10.2% when the flammability, the toxicity, the ozone depletion, the greenhouse effect and other factors of the working fluids are considered. The R601 a working fluid can be used for the high-temperature heat source, however, because of its high flammability, new working fluid should be investigated. Under the same condition, the efficiency of the organic Rankine cycle power generation system with an internal heat exchanger is higher than that of the simple system without the internal heat exchanger, but the efficiency is related to the properties of the working fluid and the temperature of the heat source. In the current study, simulations based on the engineering equation solver (EES) software are performed to determine the suitable working fluid for the simple organic Rankine cycle system in different temperature ranges. Under the condition of various temperatures and a constant thermal power of the flue gas, the influence of different organic working fluids on the efficiency of the subcritical organic Rankine cycle power generation system is studied, and its efficiency and other parameters are compared with those of the regenerator system. It is shown that the efficiency of the subcritical organic Rankine cycle system is the best when the parameters of the working fluid in the expander inlet are in the saturation state. And for the organic Rankine cycle, the R245 fa is better than other working fluids and the efficiency of the system reaches up to 10.2% when the flammability, the toxicity, the ozone depletion, the greenhouse effect and other factors of the working fluids are considered The R601 a working fluid can be used for the high-temperature heat source, however, because of its high flammability, new working fluid should be investigated. Under the same condition, the efficiency of the organic Rankine cycle power generation system with an internal heat exchanger is higher than that of the simple system without the internal heat exchanger, but the efficiency is related to the properties of the working fluid and the temperature of the heat source.