The invention relates to a device for automatically detecting an outer diameter and a ceramic core column outer diameter milling device with the device. The device for automatically detecting the outer diameter comprises a panel and a swinging seat is arranged in the middle of the panel; the swinging seat is provided with a lever with the middle part which is movably connected with the swinging seat; a pressing device for pressing one end of the lever is arranged above one end of the lever; a detection seat which is used for placing a ceramic core column contacted with the one pressed end of the lever is arranged below the lever; the detection seat is mounted at one end of the panel; the other end of the panel is provided with a stretching spring which is used for pulling down the other end of the lever at a natural state; one end of the stretching spring is fixed on the panel and the other end of the stretching spring is connected with the other end of the lever; a detection head which is used for being contacted with the other raised end of the lever is arranged above the other end of the lever; and the detection head is used for detecting a compression distance after the detection head is contacted with the lever. According to the invention, the detection efficiency of the outer diameter of the ceramic core column is improved and the accuracy of detecting the outer diameter of the ceramic core column is improved, so that the product yield is improved.