In this paper, a method to calculate the LOX/kerosene rocket engine plume infrared radiation characteristics is proposed. First of all, the simulation of engine internal flow field is carried out, and the nozzle throat section parameters are obtained as the inlet boundary condition. Then the calculation of single nozzle and multi-nozzle engine exhaust plume flow field are carried out, infrared spectral radiation characteristics and radiation imaging characteristics of the engine plume are calculated with the finite volume method (FVM) based on the field parameters. The accuracy of the method and the model are proved. On this basis, studies on the influence of chemical mechanisms and the reaction on the plume infrared radiation characteristics are carried out. It is found that the internal combustion field of the LOX/kerosene engine are simulated accurately with the multi-step chemical reaction model, the temperature is 3.34% higher than that from the thermodynamics calculation and the pressure is 2.89% greater than that from the engine test result. The reaction can enhance the infrared radiation of the plume, and the increase ratio in the 2~5 μm band of the two condition of the single-step chemical reaction and the multi-step chemical reaction achieve 50%~100% and 150%~170%, but it can’t affect the infrared spectral radiation characteristics and the variation trend of infrared total radiation intensity with detection angle. The clear infrared images are obtained with both the single-step chemical reaction and the multi-step chemical reaction, the infrared radiation intensity of the former is 90%~190% greater than that of the later, but the infrared spectral radiation characteristics and the variation trend of infrared total radiation intensity with detection angle vary widely with different chemical reactions.