The invention relates to a sand discharging adjustable mechanism of a 3D printing sand paving machine in the 3D printing technical field. According to the mechanism, the sand discharging amount can be adjustable, and the sand scraping flatness is high. The mechanism comprises a sand storage hopper and a mounting support; a sand discharging opening is formed in the lower portion of the sand storage hopper, the sand storage hopper is in an isosceles trapezoid shape along the section perpendicular to the length direction, and comprises a fixed trapezoid outer hopper and an inner hopper with the adjustable sand discharging opening width, an outer sand discharging opening is formed in the bottom of the outer hopper, connecting end plates are arranged on the upper portions of the two ends of the outer hopper, the inner hopper comprises a front adjusting plate and a rear adjusting plate, the front adjusting plate and the rear adjusting plate can vertically slide along the front and back inclined inner walls of the outer hopper, horizontal bottom plates are arranged on the lower portion of the front adjusting plate and the lower portion of the rear adjusting plate, the two ends of the front adjusting plate and the two ends of the rear adjusting plate are each provided with sand blocking end plates which are partly overlapped, when the two horizontal bottom plates synchronously and vertically slide with the front adjusting plate and the rear adjusting plate along the inclined inner wall of the outer hopper, the bottom plates can be oppositely opened and closed to form an inner sand discharging opening with the adjustable width, and the two ends of the sand storage hopper are each provided with a lifting mechanism used for driving the front adjusting plate and the rear adjusting plate to vertically slide along the inner wall of the outer hopper.