Disclosed is a conical planetary multistage large transmission ratio decelerator for slow drive transmission of an ore mill, which is composed of an input bevel gear pair arranged in a front machine body 27, an intermediate planetary gear pair arranged in an intermediate machine body, a rear planetary gear pair arranged in the front end of a rear machine body 15, and an output planetary gear pair arranged at the rear end of the rear machine body 15 which are engaged with each other and form a four-stage transmission. a driving gear 1 in the input bevel gear pair works as high speed stage input, a U-shaped rotating stand III14 in the output planetary gear pair is welded with an output shaft 18 to be a low speed stage output, U-shaped rotating stands I3 arranged on a sun gear I4 and a planetary gear I5 in the intermediate planetary gear pair form a combined floating mechanism; the central axes of the intermediate planetary gear pair, the rear planetary gear pair, and the output planetary gear pair are superposed, and the central axis of the driving bevel gear of the input bevel gear pair is vertically intersected with the central axes of the three gear pairs with a degree of 90 DEG. The uniform load effect is improved by adopting the combined floating mechanism or drum type teeth.