In recent years, it has been proposed to build Very Large Structures (VLFS) such as floating airport for effective Ocean space utilization. Safety assessment of mooring system using failure probability is one of the most important items for realizing the VLFS. In this paper, a simulation model for calculating failure probability of mooring system of a 5000 m class VLFS built within breakwaters has been proposed. And the characteristics of failure probability about some design parameters has been shown.