The invention discloses a method and a device for hardening austenitic stainless steel precisely, aiming to solve the technical problem of increasing the surface microhardness and decreasing the surface roughness of the austenitic stainless steel. The method comprises the following steps that: ion nitriding or ion carbonitriding hardening treatment is carried out to a workpiece for 1-5 hours at the temperature between 380 and 450 DEG C; the distance between an auxiliary cathode and the workpiece is between 0.5 and 10 cm, and the working voltage is between 400 and 800 V; polishing and finishing treatment is carried out to the workpiece. The device has a plurality of layers of horizontal disc-type auxiliary cathodes arranged on a main auxiliary cathode disc inside a vacuum chamber, and the workpieces are respectively placed on the horizontal disc-type auxiliary cathodes. Compared with the prior art, the method utilizes the horizontal disc-type auxiliary cathodes to place the workpieces, carries out the low-temperature ion nitriding process or the low-temperature ion carbonitriding process through the combination of temperature, vacuum degree and voltage, and then carries out the polishing and finishing treatment; a component obtained through the method has the technical effects of no deformation, microhardness more than HV0.05 500 and surface roughness Rd less than 10 nm, as well as high production efficiency.