Suction caisson have gained interest in recent years as an alternative foundation for offshore wind turbines due to its cost-effectiveness and easy installation compared to conventional foundations such as monopile. After initial penetration due to its self-weight, suction is applied until the caisson reaches the desired depth. The process of applying suction provides additional driving force due to the pressure difference between inside and outside the caisson, and induces seepage that degrades friction and tip resistance which further facilities the installation. The seepage plays vital role in the installation of suction caisson in sand; however, it might changes the soil state which affects the ultimate bearing capacity of the caisson. Several research works have been conducted to study the suction caisson installation in sand. However, the complexities of the problem including large deformation, solid-fluid and soil-structure interaction inhibited these studies from fully understanding the installation mechanism. This paper proposes a large deformation two-phase fully coupled model by using the material point method (MPM)[1]. MPM is a particle-based method that uses material points over a fixed computational mesh which makes it suitable for large deformation problems. The model takes into consideration soil-structure interaction by adopting a Coulomb contact algorithm between the caisson and surrounding soil. The effects of water seepage are included using the one point-two phase formulation and a simple penalty function is used to account for the effects of friction degradation due to seepage forces. The model can fully capture the penetration process taking into consideration the change of soil state for a more realistic simulation that provides a better prediction for penetration resistance and required suction calculations. The numerical model was validated against a field trial at Sandy Haven. Future research will focus on investigating the installation safety against heave formation, piping, and change of soil state.