The present disclosure provides a degassing membrane tube module and an online gas-in-oil monitoring apparatus for a vacuum on-load tap changer (OLTC). The online gas-in-oil monitoring apparatus includes an online monitoring housing, an oil inlet tube, a gas monitoring device, an oil tank, and an oil diffusing screen, where one end of the oil inlet tube extends through an inner wall at one side of the online monitoring housing; the gas monitoring device is located on a top inner wall of the online monitoring housing; and a front end of the oil inlet tube is provided with the degassing membrane tube module for online preliminary degassing on insulating oil. With the degassing membrane tube module, the online gas-in-oil monitoring apparatus can increase removal rates for gases in the insulating oil within a short time, and can perform degassing on various gases in the insulating oil.