Abstract: The heat insulation structure of the slug calorimeter was improved in order to enhance the measurement accuracy. The insulating sleeve with hollow structure was designed, which could excessively decrease the unstable lateral heat transfer. By designing package shell structure matched with the slug calorimeter, the heat insulation structure and heat transfer characteristics in calibration could be kept in the heat flux measurement. Numerical calculation analysis was given, which illustrates the validation of the improvement method. Based on the numerical calculation, the size of the slug calorimeter components was optimized. Then the calibration experiment was carried out. The results show that the test accuracy and precision of the slug calorimeter are improved greatly. Finally, the wind tunnel test was carried out. The test results show that the repeatability accuracy of the slug calorimeter is less than 3%. The deviation of measurement values from different slug calorimeters is less than 3% in the same fluid field. The deviation of measurement values between the slug calorimeter and the Gardon gage is less than 4% in the same fluid field.