The nonlinear interaction between the underwater explosion bubble and the free surface nearby the round bilge is studied with the bubble dynamics model based on Boundary Element Method. Firstly, the numerical bubble dynamics model under the ideal and incompressible assumption is established in this paper. Then for the overconstrain problem caused by the intersection between the rigid wall and the free surface, the traditional numerical model is improved by introducing the double nodes method to split each intersection node up to2 parts and considering the continuity conditions. With the present model, the researches on the interaction between the bubble and complex boundaries are extended. The influences of the attack angle, the initial distance of the charge and the buoyancy on the bubble and free surface dynamics are analyzed respectively. The conclusion can provide references for the anti-shock research and design of the warships.