The present invention relates to a belt tension adjustment mechanism for use with a device including a main body and a belt pulley set. The belt pulley set includes a belt. The belt tension adjustment mechanism includes a slab, at least two tension pulleys and a fixing element. A tight tension force and a slack tension force are exerted on the opposite sides of the belt to define a tight side and a slack side of the belt when the belt pulley set runs. A counterforce is generated from the tight side of the belt to exert on the tension pulley contacting with the tight side, the slab is moved in response to the counterforce. The slack side of the belt is suppressed by the tension pulley contacting with the slack side such that the slack tension force is adjustable.