Recently ocean wave energy draw much more attention for its widespread, abundant and highly energy flux density properties. Extracting energy from incident wave however, is limited for the random and unstable power input. Motion control for WEC is a promising method to improve the energy absorption and some practical applications are also verified such as latching control. In this paper, an active control strategy is proposed to achieve maximum energy capture. The mathematical description shows that the active control has the characteristic of anti-causal and the wave premonition is necessary for controller design. But the fact of premonition time horizon is still unclear. In this paper, the premonition nature is described mathematically based on hydrodynamic theory. Furthermore, a simulation is also performed to study the impacting of premonition time horizon on WEC’s properties and give a more insightful understanding of WEC active control.