This paper describes mathematical analysis of ejection dynamics as a fundamental study of subsystem for underwater vehicles. The main objective of this paper is to give a practical method to estimate maximum reaction forces during ejection process of a vehicle. A cylinder is adopted as a basic structural object of the vehicle for formulation and calculation. This paper treats an ejection process of the cylinder, which is ejected from a region in the launcher immersed in still water, to an outer region in uniform cross flow. In such a case, conventional method incorporating static equations of balance gives infinite value as reaction forces during ejection, which results in bad evaluation of the forces. Simple method of structural response analysis is adopted to evaluate approximation that the object is rigid. Under the approximation, the motion equations incorporating hydrodynamic forces are derived to perform sample computations. There are two main conclusions. The first point is that the calculated reaction forces were so reasonable that the subsystem could be realizable in comparison with that of conventional static analysis The second point is that this approach brings out the dominant parameters to the forces. The calculation results show that ejecting speed is dominant parameter affecting.