The invention discloses a vacuum brazing method for a slender weld joint of a hole-shaft matched part. The method comprises the following steps of assembling the part, and spraying brazing filler metal, specifically, spot-welding a sheared nickel sheet to a conical surface part of a shaft-shaped part, and uniformly distributing the nickel sheet at 3-5 parts along the circumferential direction of the conical surface part; assembling, specifically, putting the shaft-shaped part in a hole-shaped part and assembling to the bottom, and enabling a clamping part to be in close fit with the inner wall of the hole-shaped part; coating and injecting the brazing filler metal, specifically, uniformly coating and injecting the prepared paste brazing filler metal on the edge of a gap formed by the shaft-shaped part and the hole-shaped part for a circle by using an injector; drying, specifically, cleaning the part, putting the part into a drying oven at (80+/-10) DEG C, and keeping the temperature for (40+/-10) minutes; and conducting brazing in a furnace. According to the vacuum brazing method, the front end of the hole-shaft matched part is in close fit, and the nickel sheets with the same thickness are uniformly distributed on the conical surface along the circumference for spot welding, so that the welding clearance is doubly ensured, the conical surface is prevented from being stuck in the assembling process, and the brazing quality of the slender weld joint is better ensured; and the process is easy to operate, parts are easy to assemble, and part assembly is not affected after the nickel sheets are preset on the conical surface.