The O-ring installed on the conical sealing surface of hatch cover is in absorption state at the beginning of cover opening and there is a risk of O-ring falling off after cover opening.To avoid falling off of O-ring from the cover,O-ring absorbed motion state in the seal groove was analyzed based on finite element simulation,and critical position to the O-ring falling off was determined using the force analysis method.A calculation method of critical parameters was proposed based on Hertz contact theory,and optimal variable of O-ring to critical parameters was determined using variable sensitivity analysis method.The results show that for the hatch cover with conical sealing surface,if there is a pressure difference between the two sides of the O-ring when the underwater cover is opened,there must be an adsorption state.If the O-ring keeps moving in the adsorption state and contacts with the corners of the sealing groove,the sealing ring will fall off after the cover is opened.In order to avoid falling off of O-ring,the critical parameters of the O-ring must be considered in the selection and design of the O-ring,and the relevant parameters of the O-ring must be adjusted according to the critical parameter values.The parameter of O-ring inner diameter has the lowest sensitivity to differential pressure change,so the inner diameter parameter can be adjusted during the O-ring design process to ensure that the O-ring will not fall off.The accuracy of critical parameter’s value and reasonability of calculation method were verified by opening cover test in water.