The invention provides an automatic control mistake proofing method for nut welding of a projection welding machine. The method comprises the following steps that S1, a displacement sensor is installed on the upper electrode side of a machine head of the projection welding machine; S2, the nut welding standard height is calibrated through an upper system; S3, a nut is placed at a welding point position, and a welding starting signal is sent to a welding controller and an upper computer through a PLC; S4, the upper computer sends the welding specification parameters to the welding controller; S5, the welding controller controls the upper electrode to be pressed down according to welding specification parameters, the displacement sensor sends detected data to the PLC, and the upper computer reads the height of the upper electrode in the PLC; and S6, when the difference value between the current height of the upper electrode and the nut welding standard height is larger than a preset compensation value, it is judged that the nut is placed abnormally, the projection welding machine stops welding, and otherwise, it is judged that the nut is placed correctly, and welding is conducted. According to the automatic control mistake proofing method for nut welding of the projection welding machine, the problem that a traditional detection method is low in automation degree is solved.