A catamaran is often subjected to pitch connecting moment (PCM) when navigating in oblique sea. PCM is one of the important loads for the structural safety of a catamaran. With the wide spread of finite element analysis (FEA) system, most of the designer uses FEA for calculating structural strength of ships. However, in case of early stage of the ship design, finite element analysis is time consuming and costly expensive. So, simplified analysis is necessary in order to make repeated design analysis efficient, and to avoid errors that may occur in refined and complex analysis. This paper proposes the simplified formula to calculate the strength of cross-deck structures. In this method superstructure is not accounted for, and twin hull is treated as rigid. The simplified method was applied to two types of models. One is of 16m real catamaran and the other is 32m box-type simple catamaran. To validate the accuracy of the simplified method, three dimensional finite element analysis was conducted with FE-models of both ships. The results of simplified analysis and that of FEA was compared and it shows fair agreement. Moreover, to investigate the effect of load distribution, sinusoidal loads and concentrated four points-loads was loaded to the box-type catamaran. By comparing these results, it is found that sinusoidal load is recommended for PCM analysis to avoid the stress concentration.