Disclosed is a belt tensioning mechanism for a 3D printer and a 3D printer. The belt tensioning mechanism comprises a base, a driving pulley, at least one timing pulley, a sliding block, a tensioning pulley, a spring, a tensioning pulley cover, and a locking screw. The base is provided with a first sliding groove. The driving pulley and the at least one timing pulley are mounted on the base and are connected in series with each other via a belt. The sliding block is mounted on the base and is capable of sliding along the first sliding groove. The tensioning pulley is mounted on the sliding block and is connected in series with the driving pulley and the at least one timing pulley via the belt so as to tension the belt. The spring is configured to apply an elastic force to the sliding block. The tensioning pulley cover is fixedly mounted on the base and is provided with a second sliding groove corresponding to the first sliding groove. The locking screw passes through the second sliding groove and is in threaded connection with a threaded hole in the sliding block, such that the sliding block is locked and fixed relative to the tensioning pulley cover.