To simulate the ship advancing in large waves with large-amplitude motion, a CIP-based strongly-nonlinear seakeeping analysis tool is developed. The effects of viscosity and strongly-nonlinearity are considered in the numerical model, which is based on the viscous flow theory. First, convergence studies are carried out by calculating the radiation problems of S175 hull with forward speed. Then, the ship response of S175 hull including the heave and pitch motions in waves with amplitude and length of waves varied is calculated. Finally, strongly-nonlinear ship responses in large head waves are calculated and captured successfully. It is confirmed that the developed seakeeping analysis tool has numerical stability and accuracy, and has the capability of simulating the strongly-nonlinear seakeeping problem, such as, the phenomena of wave breaking, slamming, and green water on deck.