Abstract: The startup and operation transients of alkali metal high temperature heat pipes were studied in this paper. Based on the traditional heat pipe frozen startup model, a three-stage model depending on vapor flow transition was established, and a heat pipe startup transient analysis code, HPSTAC, was developed. The relative deviation between the code simulation result and experimental value is less than 15.7%. The HPSTAC code was used to perform the startup transient of single sodium-potassium heat pipe and sensitivity analysis. The results show that 450, 660 and 1 550 s after the starting, the heat pipe enters the second, third, and quasi-steady states respectively, and the vapor region tends to a consistent temperature of 837 K. The startup surrounding temperature mainly affects the heating rate of the condensation section, and there is a threshold on the heat flux influence of stage process during heat pipe startup.