Based on the nonlinear finite element software ABAQUS,we analyze the current force of mattress in ultra-deepwater and give the simplified formula. Comparing the horizontal force and vertical force at the end of the platform trap door,we obtain the results that the horizontal force can be estimated by the simplified formula and the vertical force can be reckoned by the difference between the gravity and buoyancy of mattress in the engineering application.In addition,it is necessary to adjust the ship ballasts in the process of the geotextiles-laying to balance the hell angle caused by the vertical force.