The invention provides an interface conversion device. The device comprises an upper crossover flange with a pressure testing interface, and a lower crossover flange with an opening; a joint is arranged in the opening, and an axial through hole is formed in the joint; the upper end of a sealing member made of flexible elastic materials is fixedly connected with the lower end of the upper crossoverflange; a central channel is formed in the sealing member, and the central channel is communicated with the pressure testing interface and the axial through hole; the side wall, close to the middle,of the sealing member necks inwards, so that the central channel is provided with an upper channel section close to the upper end, a lower channel section close to the lower end and a middle channel section close to the middle and located between the upper channel section and the lower channel section; the inner diameter of the middle channel section is smaller than the inner diameter of the upperchannel section and the lower channel section. According to the device, crude oil which adheres to the outer wall of a hollow short piece can be scrapped out while the occurrence of oil and gas leakage at a sealing wellhead is prevented to prevent environmental pollution of a well site caused by dropping of the crude oil on the ground.