The invention discloses an adjustable transmission mechanism. The adjustable transmission mechanism comprises a transmission platform and a belt, and further comprises U-shaped supports. The U-shaped supports are fixed to the two ends of the bottom of the transmission platform respectively. Kidney-shaped holes are formed in the bottoms of two legs of each U-shaped support respectively. A roll shaft is arranged between the two legs of each U-shaped support. First threaded holes are formed in the centers of the two ends of each roll shaft. A fixed frame and a movable frame are arranged on the outer sides of the two legs of each U-shaped support respectively. The fixed frames are fixed to the legs of the U-shaped supports. The movable frames are fixed through screw threads. A second threaded hole is formed in one side face of each movable frame. The roll shafts are fixed by the movable frames through bolts. The belt wraps the outer side of the conveying platform and the outer sides of the roll shafts. The tightness of the belt is regulated by vertically regulating the height of the roll shafts through screws, and the problems that the belt gets loose and can not be used are effectively solved, the service life of the belt is prolonged, cost is saved, and meanwhile the adjustable transmission mechanism is simple in structure, convenient to use, low in manufacturing cost and suitable for practical application.