Transmission efficiency is a key characteristic for gear box,so a test system is built to measure gear box’s transmission efficiency.But in the test process,some wrong test results may be gotten by using ideal mathematic formula which is based on the law of conservation of energy.The main interference factor was that there existed some energy storage devices in this test system.In order to get correct transmission efficiency,a new calculation method was used to exclude out the interference factors from the mathematical calculations.With this new method,the experiment process was repeated and then a good result was gotten,which illustrated the correctness of the analysis and mathematical calculations from another side.