The invention discloses a multifunctional fair metering system and metering method based on an LNG tank car, and belongs to the technical field of metering. The system comprises a first valve, a liquid storage tank pressure gauge, a liquid storage tank, a liquid storage tank liquid level meter, a second valve, a gasifier, a third valve, an online chromatographic analyzer, an in-tank liquid level meter, a temperature transmitter, a pressure transmitter and a flow computer. According to the invention, multiple metering modes of volume metering, mass metering and energy metering can be realized, accurate metering can be ensured, trade disputes are reduced, the working efficiency is improved, and the device can be widely applied to the fields of trade handover of liquid natural gas (LNG), liquefied petroleum gas (LPG), ethane (C2) and the like.