Abstract: The leak in a manned spacecraft on orbit will lead to disaster, so the leak should be tested and located in time, and the ultrasonic leak detection is an effective method to solve the problem. The generation of ultrasonic waves in the spacecraft leak is analyzed and the center frequency is calculated. Based on the ultrasonic attenuation theory, experiments are carried out and the obtained data are used to fit the attenuation curves and equations of four different leaks. And the verification test is used to validate the fitted curves. It is shown that, in the range from 20 mm to 500 mm, the acoustic pressure level decreases according to the first order exponential decay model and increases with the leak aperture diameter. This paper provides some basic ideas of the on-orbit ultrasonic leak detection system.