When the air gap of the column stabilized units turns out to be negative, the evaluation for the consequences of the negative air gap is needed, which includes the calculation of horizontal wave impact loads. According to the method of engineering design and the related international codes, a practical method is proposed for the engineering design of the column stabilized units. As a demonstration, the forward bulkhead of a certain column stabilized unit is analyzed. The wave impact loads on the local structure is calculated and the transient simulation is performed by means of the finite element method (FEM). Finally, the strain and deformation in the forward bulkhead under the horizontal wave impact loads are obtained. The results show that the effect of horizontal wave impact loads on the vertical surface of deck box cannot be ignored. Thus, once the air gap of the column stabilized units is negative, the horizontal wave impact loads on the vertical surface of deck box must be fully considered. The local structure should be reinforced and re-analyzed based on the simulation results until the design meets the rules and standards.