Abstract: Objectives Aiming at the problems of limited modeling ability and insufficient applicability of complex ship types in the two-dimensional slicing method based ship unsinkability calculation method, a surface element method based ship unsinkability calculation method is proposed based on the three-dimensional model of fully attached ships. Methods Adopting a triangular panel discrete three-dimensional model, setting the normal direction of the ship and damaged compartment panel opposite, so that the buoyancy at the damaged compartment cancels out each other and causes buoyancy loss; Using waterline surface segmentation algorithm to achieve matching of wet surface elements in any floating state; Based on the Gaussian formula, the integration of drainage volume is transformed into the integration of wet surface area, and the drainage volume and waterline surface elements are further solved through the panel projection integration method; Using Newton's iterative method to calculate the floating state of damaged ships, and based on this, conducting unsinkability assessment. Results Taking the Wigley numerical ship model and the ONRT standard inner capsized ship model as examples, the accuracy of the algorithm was verified, and a unsinkability evaluation was conducted for the ONRT fully attached ship model.Conclusions The calculation example shows that the method has reliable calculation accuracy, strong applicability to ship types, and practical engineering value.