Ocean wave energy is one of the most abundant and widely distributed renewable energy resources, but it has not been extensively used so far due to the cost and efficiency issues of typical wave energy converters (WECs) based on electromagnetic generators. In recent years, the rapid development of triboelectric nanogenerator (TENG) technology provides a highly promising solution for cost-effectively harnessing the low-frequency ocean wave energy. In this work, we report a novel WEC design based on the multi-grating triboelectric nanogenerator (MG-TENG) model. It is shown that the designed MG-TENG is able to deliver over 140 V open-circuit voltage and 60 μA rectified short-circuit current, respectively, which is enough to power over 60 commercial LEDs. What's more, its average charging current achieves 12.3 μA, which is better than most reported TENG models. Particularly, instead of sparse pulse signals generated from those TENG models in vertical contact-separation mode, the MG-TENG works in a freestanding triboelectric-layer mode which is able to provide more continuous and smooth power outputs, thus is more suitable for the low-frequency wave energy harvesting. Moreover, the MG-TENG based wave energy converter is designed, fabricated and tested in a wave tank under different wave conditions. It is observed that the instantaneous power output reaches 54 μW and the instantaneous maximum power output density arrives at 4.2 mW/m2 under the 8 cm and 0.5 Hz wave condition, exhibiting strong power output capability. It can be expected that array installation of MG-TENG WECs is a potential solution for the power supply problem in marine environment.