The objective of paper is to develop a stochastic method to predict the slamming load acting on a very large pontoon-type structure with shallow draft in irregular waves. Threshold values of relative wave height and its velocity are important parameters to predict the slamming load acting on ships in irregular waves. In the case of the very large pontoon-type structure, however, it was difficult to determine the threshold values because the values were highly influenced by the rigidity of structure. This problem were related to development of a new stochastic method for prediction of slamming load without threshold values. Using the new stochastic method, the probability of occurrence of the slamming pressure acting on the very large pontoon-type structure were predicted.