The invention provides a device for fixing a nut automatically. The device is used for carrying out nut fixing on a bottom disc of a compressor. The device comprises a nut fastening mechanism, a machine frame, a locating plate and a main control computer. The locating plate comprises a position sensor which is electrically connected with the main control computer. The locating plate is used for fixing the bottom disc. The nut fastening mechanism is movably connected with the machine frame and moves on the machine frame, and the nut fastening mechanism is used for adjusting a working position. The main control computer is electrically connected with the nut fastening mechanism which comprises an operation end and a lower pressing air cylinder. The operation end is electrically connected with the main control computer. The lower pressing air cylinder is in transmission connection with the operation end. The lower pressing air cylinder is electrically connected with the main control computer. The design is reasonable, the structure is simple, a compressed nut of the machine bottom disc can be fixed automatically, the automation degree is high, accordingly, human cost is greatly saved, production efficiency is improved, and nut fixing consistency is guaranteed.