With the increasing of human space activities, the number of space debris has increased dramatically, the possibility that spacecraft in orbit is impacted by space debris is growing. It is important to detect and locate the gas leak accurately and timely. In this paper, a leak detection method using ultrasonic sensor array is proposed. Firstly, the ultrasonic sensor array is used to detect the leak acoustic signal which propagates as Lamb wave through spacecraft structure. Then we apply beam forming algorithm to determine the direction of the leak source. Finally, we use cross section method to locate the leak source. The result shows that, the leak location error is less than 90mm in one square meter plate when the diameter of leakage hole is larger than 1mm. This method can meet the needs of leak detection for the spacecraft in orbit.