The invention discloses a speed limit protection device with invariable braking force. A disc brake is composed of moving type friction discs and fixed type friction discs; and adjustable pressing force is applied between each fixed type friction disc and the corresponding moving type friction disc through a spring to enable the disc brake to acquire a constant brake torque. When lifting equipment runs at an overspeed, and the speed reaches a speed limit protection device trigger speed, a centrifugal block in the speed limit protection device is in contact with a brake bead or a brake groove arranged on the inner side of the shell and is locked and stopped after being thrown away by the action of centrifugal force to enable a centrifugal seat to stop rotating, and the fixed type friction discs connected with the centrifugal seat also stop rotating. The moving type friction discs rotate relative to the fixed type friction discs to obtain a brake torque, a brake gear is connected with the moving type friction discs through a spindle of the speed limit protection device to generate the braking force, and the construction lifting equipment is decelerated and stops. The speed limit protection device is of a simple structure, is reliable in performance, convenient to use and lower in cost and is suitable for freight construction lifting equipment or simple lifting equipment with narrow and small mounting space, inconvenience in reset operation and lower running speed.