A rotor external diameter detection device disclosed by the present invention comprises a [-shaped fixing support, opposite measuring bases are arranged at the upper and lower ends of the fixing support respectively, and the measuring base at the lower side of the fixing support is fixed on the fixing support, and a plurality of guide columns are fixed on a bottom plate of the measuring based at the upper side of the fixing support and are plugged in the fixing support. A cylinder is fixed on the upper end face of the fixing support, and a piston rod of the cylinder penetrates the fixing support to be fixed on the bottom plate of the measuring base at the upper end of the fixing support. According to the present invention, an electromechanical device is used to substitute for the manual artificial detection of the external diameter of a rotor, thereby not only being convenient to detect the rotor, improving the detection efficiency, but also positioning the rotor effectively, and detecting the external diameter accurately.