The utility model discloses a tooth side gap detection mechanism. The tooth side gap detection mechanism is characterized in that a friction wheel is set; the outer circumferential surface of the friction wheel abuts against the outer circumferential surface of a driven gear of a main retarder; the friction wheel can drive the driven gear of the main retarder to rotate back and forth in a rotation angle interval A of a tooth side gap in a friction manner when a driving gear of the main retarder is locked; a measurement wheel is set, the outer circumferential surface of the measurement wheel abuts against the outer circumferential surface of the driven gear of the main retarder so as to detect the value of the rotation angle interval A; and the friction wheel, the measurement wheel and the driven gear of the main retarder are located at the same vertical plane. According to the tooth side gap detection mechanism of the utility model, when the driving gear of the main retarder is locked, the friction wheel, adopted as a driving wheel, drives the driven gear of the main retarder, and the measurement wheel, adopted as a driven wheel, detects the rotation of the driven gear of the main retarder, and therefore, automatic detection of the tooth side gap can be realized, and production efficiency on an assembly line can be significantly improved, and accuracy can be improved.