Abstract: To accurately measure the out-tube motion parameters of an underwater launcher during launch testing of various undersea vehicles, a method for accurately describing the out-tube motion parameters of an undersea vehicle in launching was presented by using inertial measurement unit (IMU). The basic principle is that the accelerometer in the IMU module directly measures the real-time acceleration signal of the motion process, and the speed and displacement can be obtained indirectly through the numerical integration operation of acceleration. To obtain more accurate acceleration measurement, the gyro in the IMU was utilized to measure the deflection angle of each axis in the course of the launching motion of the undersea vehicle, and the deflection angle was used to remove the gravity component of the collected acceleration signal and to eliminate the acceleration measurement error. Then, the direct-current (DC) removal and the trend term preprocessing were performed on the collected acceleration signal to further improve the accuracy of the numerical integration operation on the acceleration signal to obtain the movement speed and the displacement. Based on the above theoretical analysis, test system was set up and repeated tests were conducted. The results show that this method successfully realizes the accurate description of the out-tube motion parameters of an undersea vehicle in launching.