The present invention is to install a transmission belt between two noncircular synchronous transmission pulleys, thereby providing a non-sliding synchronous transmission feature between the belt and the pulleys, when the noncircular active pulley is driven to synchronously link the noncircular passive pulley to rotate through the transmission belt, a noncircular synchronous transmission pulley set having periodically varying speed ratio and circumference compensating function is formed, through the two pulleys being formed with certain noncircular wheel shapes, during the synchronous transmission, the circumferences of the two wheel shapes can be mutually compensated.