Abstract: The ramjet reactor has excellent characteristics of large specific impulse and high power density,and has broad application prospects in the field of aerospace propulsion in the future. Under normal operating conditions,the reactor needs to complete various transient operating conditions with rapid power response,and complete the startup and shutdown process in a very short time. The use of traditional numerical methods to study its transient three-dimensional nuclear-thermal coupling characteristics has great difficult. Based on the open source OpenFOAM platform, this paper developed a three-dimensional transient nuclear-thermal coupling solution program,proposed a new fluid-solid coupling boundary,and realized the rapid prediction of the flow and heat transfer characteristics of the solid core and the flow channel. The point kinetic model was employed to solve the core neutron physical properties.Through the coupling of the physical module and the thermal module,realize the numerical analysis of the flow and heat transfer characteristics of the gas reactor core during the transient process. The simulation results show that while the power of the ramjet reactor increases rapidly,the temperature of the solid material will not rise fast,but will rise steadily until it reaches a steady state. The reactor is still safe during the startup process when the instantaneous thermal power exceeds the design benchmark value.