In order to explore the operation characteristics of nuclear power conversion system with open-air Brayton cycle coupled with heat pipe reactor when the core power and load change,the system simulation model is established based on Modelica language,including the sub-models of the reactor core,the heat pipe and the Brayton cycle,and the accuracy of each model is verified.The transient simulation and analysis of loss of load(LOL)and power increment and reduction processes are carried out by using the established model.The calculation results show that in the transient process,the change of load or core power will lead to the change of rotating speed,and it is necessary to control the turbine flow through the bypass control valve to restore the rotating speed to stability.Under LOL condition,the core temperature will drop,and the reactivity feedback will increase the core power by 2.3%and the maximum fuel temperature by 1.7 K.During the reactor power increment and reduction,the peak normalized core power caused by reactivity feedback is 102.6%and 100.7%respectively.The results of this paper provide a reference for the safety analysis of the heat pipe reactor coupled with open-air Brayton cycle.