Abstract: The turbopump-fed rocket engine used for the upper stage is complex in structure and has a limited flight time, which poses a challenge to the test validation of their thermal design. In view of to the characteristics of the engine, a transient thermal test method is proposed, with a regulation of the heat flux on the complex outline surfaces of the engine, to build up a thermal analysis model of adiabatic flux sensors (“black-plates”) for each sub area. The sectional stepwise heat flux loading strategy is applied in the thermal test to simulate sunshine and eclipse, respectively. The thermal design and the thermal analysis model of the engine are validated in accuracy by the temperature measurement results. Moreover, on the basis of the current thermal analysis model, the initial temperature is corrected for a better prediction of the temperature of the engine in flight.