The invention relates to a special vacuum totally-hermetic oil sampling method for a main transformer butterfly valve and provides a special vacuum totally-hermetic oil sampling apparatus for the main transformer butterfly valve. The apparatus comprises a main transformer butterfly valve connecting flange, an oil storage cavity, a vacuumizing vent valve, a miniature vacuum gauge, a sampling valve and an oil level observation window. The method adopts the principles of a communicating device, reasonably utilizes the opening space of a valve plate after closing of the main transformer butterfly valve, removes air in the space by vacuumizing and employs a combination of the oil pressure of a main transformer and vacuum negative pressure to realize accurate control of the oil output amount of the main transformer butterfly valve and totally-hermetic oil sampling. The oil sampling apparatus and method provided by the invention are easy to install and use, have good sealing performance and can perform vacuum totally-hermetic oil sampling on the main transformer butterfly valve.