A mixer for dental molding powder has a motor to drive a holding cup pivotally located on a chassis to rotate. A regulation device is provided that has a bolt and a screw bar to hold a pulley on the chassis. By use of a plurality of belts, the pulley and rotary wheels located on two outer sides of the chassis the holding cup can be driven to rotate. The pulley can be moved by adjusting the regulation device so that the belt can be loosened from the pulley for easy and fast replacement.