A feeder with variable rhomboidal walls includes a base housing, a soft hopper, a plurality of extrusion mechanisms, a pusher screw pole, a motor, and a decelerator. The extrusion mechanisms are respectively provided at four appropriate positions of the soft hopper, and are activated by protruding wheels with the aid of the motor in conjunction with the decelerator, so that the four extrusion mechanisms operate in pairs to squeeze the soft hopper while rendering a rhomboidal space change. Accordingly, a bridge effect associated with the prior art is eliminated to allow a raw material to fall and then to be dispensed by the pusher screw rod for further processing.