Abstract: Canister exit velocity is a key parameter for the launch and flight control of loitering munitions, which directly determines their attitude stability during canister exit, trajectory accuracy, and effective range. When a loitering munition is launched from the water surface, the launch canister exhibits a vertical downward motion, resulting in a certain deviation in the canister exit velocity of the loitering munition compared with that in land-based launch. Aiming at the problem of canister exit velocity deviation of surface-launched loitering munitions, this paper conducts force analysis and motion characteristic analysis on the loitering munition and launch canister respectively, and establishes motion models for loitering munitions launched from land and water surface. The study reveals that, compared to land-based launch, the in-canister velocity and exit velocity of loitering munitions are both lower during water-based launch, with a shorter acceleration duration within the canister. This study provids a theoretical basis and computational method for the engineering application of surface launch systems.