Abstract: Heat pipe has been widely used to cool the space nuclear reactor core. Flow in the reactor heat pipe, which is characterized by the evaporation and condensation of its inner fluid, is remarkably different from the tube flow, thus the variable mass flow, axis velocity and radius velocity must be taken in account in its calculation. In this paper, a study was made on the calculation method of reactor heat pipe vapor flow’s pressure, temperature and velocity profile, and a computer code SNPS-HPD was developed to calculate the space nuclear reactor heat pipe vapor flow. The reliability of the code SNPS-HPD was verified by the data from sodium heat pipe experiment first. It is also proved to be well in a check calculation of, by comparing the result with that from reference, the vapor flow in the HP-STMCs Space nuclear reactor high temperature lithium heat pipe under various operating conditions. All that approved the employment of the SNPS-HPD code in the space nuclear reactor heat pipe design.