The IR radiation characteristic of exhaust plume of the liquid rocket is numerically calculated. Based on the established HITRAN database and the numerical simulation results of the exhaust plume field, the radiation transmission equation is numerically solved. The absorption and radiation of the atmosphere are considered. The calculation results indicate that the transmission conditions, height and angle, exert important influence on the distribution of spectrum intensity. The spectral irradiances achieved by solving the RTE are compared with the test data, which show that the calculation method is feasible and the solution is proper.