An apparatus for wave energy harnessing includes at least one slide shaft, a buoyant unit, an energy-transmitting unit, and a rotate unit. The slide shaft is firmly mounted on the seabed. The buoyant unit is movably engaged with the slide shaft and has a reservoir and an adjusting board, so as to control the vertical position of the buoyant unit relative to sea level. The energy-transmitting unit is mounted on the buoyant unit. The rotate unit rotatably engages with the energy-transmitting unit and connects with an energy-transfonning unit through a transmission shaft. Thereby, the buoyant unit is driven by waves to vertically slide along the slide shaft, the energy-transmitting unit rotates the transmission shaft of the rotate unit, and, thus, the energy-transforming unit generates power.