In order to improve the accuracy of the test for boron powders combustion heat and determine the main factors affecting the uncertainty of boron powders combustion heat test under the combustion-supporting condition, the direct assessment method was adopted to analyze the uncertainty based on boron powders ignition and combustion characteristics as well as the principle of boron powders combustion heat test under the combustion-supporting condition. The impact degree of various factors on uncertainty of boron powders combustion heat value testing were calculated. Results show that the main factors affecting the uncertainty for boron powders combustion heat test are the temperature rise of the calorimetric system, the calorific value of combustion-supporting agent, and the mass of boron powders involved in the combustion. Reducing the uncertainty of calorific capacity of the calorimetric system, can increase the accuracy of measurement for boron powders combustion heat value. The relative uncertainty of boron powders combustion heat value test using combustion-supporting agent is only 0.95%.