The constant volume burner is a kind of test method to measure the burning rate of solid propellant under high pressure. Heat loss of the constant volume burner is one of the most important factors affecting the test accuracy of burning rate. Heat loss rate equation was derived to study the various factors on the rate of heat loss. The assessment method of the uncertainty of heat loss rate has been established, and the uncertainty also has been obtained. According to the study, it was known that the sample propellant burning surface, the gas constant volume specific heat, gas pressure, gas temperature are main factors of the uncertainty of the heat loss rate, and burning surface is the most important factor. The precision of propellant sample size should be increased to reduce the deviation of propellant burning surface of the sample.