The effect of gravity often costs time and effort in stability calculation in combination of multi-load case and multi-scheme.An algorithm based on the computation geometry is introduced to simplify the calculation of the gravity structure’s stability.The design section is cut to sub-polygons by water level after its convexity concavity is judged.The area and barycenter of sub-polygons are calculated respectively considering whether the concave point insists in it.Then it is summarized to deduce effect of gravity and safety coefficient.A set of computer program is compiled based on the above algorithm，which is tested and verified by the application in a ship lock project.