The heat pipe cooled reactor adopts the passive heat transfer technology, which has fast thermal response and can avoid the single point failure of the reactor core. It has the characteristics of high power density, long life, strong environmental adaptability and stable working performance, and is currently a hot research topic of space nuclear reactors. Based on the reactor Monte Carlo neutron transport code RMC (reactor Monte Carlo code) developed by Tsinghua University, taking the heat pipe cooled reactor INL Design A designed by Idaho National Laboratory (INL) as the research object, three kinds of heat pipe working fluids were selected to carry out the physical calculation of the heat pipe cooled reactor core. The calculation results show that the lithium heat pipe working fluid not only has good thermophysical parameters, but also has the largest delayed neutron effective fraction, the harder neutron energy spectrum, the smallest burnup reactivity loss and the best proliferation performance in the heat pipe cooled reactor with lithium heat pipe working fluid, which is conducive to the miniaturization and long life of the heat pipe cooled reactor core. Therefore, it is recommended as heat pipe cooled reactor heat pipe working fluid. [ABSTRACT FROM AUTHOR]