The invention discloses an assistant device for automobile detection equipment in the industrial field of automobiles, and in particular relates to a wheel constraining device for detecting braking force of an automobile rack. The invention aims to solve the problem that the braking force detection error is great because the detected automobile is in an unstable state in braking force detection of existing automobile rack. The device consists of a constraining cylinder mechanism, a lifting hydraulic system and a positioning hydraulic system. Horizontal forward constraint is applied to the wheel tested by the constraining cylinder to test that the wheel is in a stable state. The lifting and positioning hydraulic systems are connected through a connecting arm in a herringbone shape, so that the constraining cylinder can move freely in a two-dimensional plane.