Oscillating Water Column (OWC) is widely used in the wave energy conversion, which can convert wave energy into pneumatic energy through the bi-directional air flows. The traditional study on OWC system always focused on two separated phases, respectively. This paper proposed the predicting method to evaluate the operating performance of Jeju pilot OWC system in Korea in real sea conditions, which has induced the interaction between the two energy converting phases. The real sea conditions are considered as the combination of regular waves to estimate the accurate hydropower from incident waves. The estimating method for individual regular waves is based on the numerical simulation of the OWC chamber within effects of turbine effects. The computational method has been validated by corresponding experimental data. The numerical wave tank using computational fluid dynamic commercial software is employed to predict both the free surface elevation inside the chamber installed with orifice device and the reciprocating air flows in the duct. Numerical simulation of impulse turbines employs the multiple reference frame method to simplify the rotation as the steady conditions.