An investigation on smeared thickness method used in prediction of grounding characters for double bottom tanker over obstacles with large contact surface and trapezoidal cross-section was conducted in this paper. Present analytical methods predicting grounding characters mostly simplify the stiffeners’ effect, and consider it by smeared thickness method. Paik proposed a method to calculate the smeared thickness of the plating on the results of model tests of axial crushing of square tubes. This method is used frequently for its simple calculation work. In his method, Paik calculated the smeared thickness by adding a parameter k to take the property of stiffeners on bottom plating into account, and took it for granted that the parameter k can be used as 1.0. However, taking the parameter as 1.0 is not sufficient to consider the stiffeners’ roles during the sliding grounding course, which has been proved by Hu. What value should the parameter k be? It is answered in this paper. Numerical simulation technique is used to do the investigation. A hold of a double bottom tanker is chosen as the object. Two finite element models are built, one with stiffeners and the other without stiffeners but with smeared thickness on plating. Structural components within the double bottom are built with fine mesh, and the rest are built with coarse mesh. LS_DYNA is used to do the numerical simulation. Grounding resistances and energy dissipation are obtained, to do the comparison and verification. Cases with different indentations are defined. The results of the simulations for the model with stiffeners are taken as true values. The results of simulations for the model without stiffeners but with smeared thickness plating are used to do the comparison, on purpose of determining the value of the parameter k. With the comparison, the feasibility of the smeared thickness methods is verified. The suitable values of parameter k in smeared thickness method are determined for different indentations cases, for the double bottom tanker during sliding grounding scenario with seabed obstacle with large contact surface and trapezoidal cross section.