The invention discloses a brake excircle runout detection mechanism, and relates to an excircle runout detection mechanism, which comprises an expansion core positioning mechanism, a runout detection mechanism, and a taper sleeve swinging base. The expansion core positioning mechanism is connected with an inner hole of a work piece, and the runout detection mechanism is contacted with the excircle of the work piece, and in addition, the bottom part of the expansion core positioning mechanism is disposed on the taper sleeve swinging base. The expansion core positioning mechanism comprises a fixed core pressing plate, a spring expansion sheet, a positioning bottom plate, and a core shaft. The fixed core pressing plate and the positioning bottom plate are sequentially connected with the core shaft in a sleeved manner, and the positioning bottom plate is fixedly connected with the core shaft by a fastening member. The spring expansion sheet is disposed between the fixed core pressing plate and the positioning bottom plate, and the upper end of the core shaft is fixedly disposed on the fixed core pressing plate after being pressed tightly by the fastening member. The brake excircle runout detection mechanism is used to eliminate a gap between the positioning core shaft and the brake bottom plate, and has advantages of high detection precision, high production efficiency, reliable structure, convenient and fast use, wide application range, and easy popularization and application.