This paper presents a new wave energy converter (WEC) concept with variable stiffness mechanism (VSM). The WEC system includes four main components: a frame structure; a buoy with guiding shaft; a VSM with a ball screw, two pre-tension springs, motor and gearbox; a supplementary mass coupled with linear generator. In the first embodiment, the up and down motion of the buoy under heave wave is converted to linear motion of the supplementary mass. The mass will oscillate two coils coupled with it. The oscillation of two coils in two magnetic walls will create electricity by the principle of electromagnetic induction as a linear generator. In the second embodiment, two pre-tension springs will create a negative stiffness effect acts on the oscillation of the mass thus its motion is amplified. In the third embodiment, the VSM with the motor can control the motion of the ball screw thus the displacement of the spring will be changed to increase the restore spring force which can boost the motion of the mass. The specification of the system is proposed and studied. The working principle is analyzed. The mathematical model is then derived to investigate the system operation. Simulation has been done to evaluate the operation of the system.