The invention discloses a heavy load internal gearing drum worm transmission reduction gear. The heavy load internal gearing drum worm transmission reduction gear comprises a box which includes a box base and a box lid. The box is provided with an inner gear, one end of the inner gear is provided with an end hub output shaft which protrudes from the box, and the drum worm on an input shaft of the drum worm engages with the inner gear. The input shaft and the output shaft are in a non-orthogonal staggered arrangement, and the drum worm is arranged inside the inner gear and engages with the inner gear. A tooth face of the drum wear is enclosed by a tooth face of the inner gear and is in the shape of a drum which is big in center and small in two ends. The heavy load internal gearing drum worm transmission reduction gear has a multi-tooth contact of an enveloping torus worm reducer and has the advantages of being high in carrying capacity, good in lubrication effect, long in service life and easy to manufacture. The internal gearing of the drum worm and the inner gear enables the structure to be more compact, smaller in volume, and lighter in weight; therefore, the heavy load internal gearing drum worm transmission reduction gear is small in volume and light in weight.