The utility model discloses an adjusting structure for the tightness of a driving belt of a cutting wheel of a four-side planing machine, which comprises a swinging arm, a belt idle shaft and a force output mechanism, wherein, one end of the long swinging arm is pivoted on a machine table; the other end of the long swinging arm is pivoted on the rotary belt idle shaft; the force output mechanism is arranged in the center of the swinging arm; a compressible spring is arranged in a cylinder; both ends of the spring are respectively abutted on the piston end surface of the cylinder and the inner end surface of the cylinder; the inner end surface of the cylinder is near to a fixing end; compressed air is injected into the cylinder. The adjusting structure comprehensively utilize the characteristics of the spring and air pressure. Thereby, the utility model has the advantage that the utility model can effectively improve the tensile stability of the driving belt. Besides, the utility model has the advantages of energy saving, high production efficiency, etc.