Sensor capabilities of underwater-guided weapons are gradually improving in modern society. However, various problems have been reported owing to the instability in the initial posture of the launch, which is mainly caused by the influence of the underwater environment and cavitation. This study aimed to confirm the initial behavioral characteristics of underwater decoys ejected from submarines. The tests were conducted in indoor tanks of a limited size. A test was conducted and compared with a launch while moving the launch pad to a stationary state. Because of the test, it was confirmed that an external force was applied on the y-axis when the underwater vehicle was fired while moving. This means that there iwas an error in calculating the roll direction in the driving performance of the underwater vehicle, which is helpful in improving the initial behavior. Based on the test results, the initial driving characteristics of underwater vehicles are expected to improve.