In order to measure the gas leakage flow rate conveniently and rapidly, an invasive ultrasonic measurement method was proposed. A function was established to describe the relationship between average power of ultrasonic signal generated by a gas leak and upstream pressure of the leak, net sectional area of the leak and the distance between the leak and ultrasonic sensor through experiment. In practice, the net sectional area of the leak was calculated by the average power of ultrasonic signal received by an ultrasonic leak detector through the function and the leakage flow rate was obtained then. Experimental results indicate that error of the leakage flow measurement method is less than 9%, the method satisfies accuracy requirement of leakage flow rate measurement, and it can provide a convenient, low cost and high efficient flow rate measurement method for diagnosis and evaluation for energysaving in pneumatic systems in plants and other applications.