Abstract: The launch process of submarine-launched missile is divided into three stages: barrel-exit stage, underwater motion stage, and water-exit stage. The motion and posture of the missile after the first barrel-exit stage directly influence its motion response after entering the water and exiting the water. Moreover, due to the interaction between steam and missile and water, the launch problem also involves multi-physics field coupling. Therefore, constructing a multi-degree-of-freedom model for missile launch and conducting research on the hydrodynamic response characteristics during the barrel-exit process have both scientific and engineering value. In response to the strong nonlinearity caused by the multi-physics field and multi-degree-of-freedom motion coupling during the barrel-exit stage, a 2D numerical model is established for simulating underwater launch based on a multi-degree-of-freedom motion model, a cavitation model, and a collision model. Firstly, the validity of model is verified by comparison with launch experimental data in still water. Then, research on the hydrodynamic response characteristics of a missile in crossflow is conducted. Comparison with the single-degree-of-freedom numerical model out of the barrel shows that the multi-degree-of-freedom model can well show the influence of crossflow well at different crossflow velocities; high-speed crossflow will cause the collision between the missile and the launch barrel, and the faster the flow rate, the sooner the collision will occur. The collision will make the missile unable to exit the barrel or its attitude will deteriorate after exiting the barrel; and the multi-degree-of-freedom model can also show in detail the changes in the backflow of seawater inside the barrel.