Abstract: An adjustable positioning structure for a feeding support plate of a circular sawing machine contains a magnetic element, a micro pneumatic cylinder, and a control button. The magnetic element is accommodated in a magnetic slider which is fitted with a guide column on a material feeding side of a table of the circular sawing machine and is configured to produce a change of a magnet value of the magnetic slider when the magnetic slider moves along the guide column reciprocately, such that a displacement value of the magnetic slider is calculated and is displayed by a display on the magnetic slider. The micro pneumatic cylinder is mounted on the magnetic slider and is configured to fix or release the magnetic slider relative to the guide column. The control button is arranged on the feeding support plate and is configured to control the micro pneumatic cylinder to operate.