Abstract: The slug calorimeter, the improved slug calorimeter and the water calorimeter are usually chosen to measure the heat flux values on the model surface based on the magnitude of heat flux values. There inevitably exists heat transfer between the thermal insulation sleeve of the calorimeter and the model, or pyrolysis of the thermal insulation sleeve, either of which can result in measurement errors of the heat flux. In this paper, the initial cold wall heat fluxes were converted to the hot wall heat fluxes at different moments, which were used as the new boundary conditions for unsteady heat transfer calculation at the corresponding moments. For the slug calorimeter and the improved slug calorimeter, the flux values were obtained based on the temperature change at different moments. For the water calorimeter, the flux values were obtained based on the temperature difference between inlet and outlet water at the stable time. The errors were analyzed by comparing the calculated heat fluxes with the cold wall heat fluxes at the initial moment, which can be used to guide the flux measurement in thermal protection tests.