The invention provides a bottom die body of a wheel die. Grid lines are arranged in the areas, corresponding to spokes, of the surface of a die cavity of the bottom die body and include the radial grid lines and the circumferential grid lines, and the depths of the grid lines are 0.5 mm-1 mm; and the included angles between the radial grid lines and the circumferential grid lines are obtuse angles, and the grid lines are evenly arranged at the intervals of 3-5 mm. Sawtooth-shaped annular grooves are formed in the surface of a back cavity of the bottom die body and evenly distributed in the radial direction; and cooling air pipes are arranged in the back cavity of the bottom die body, and air outlets are formed in the air pipes and face the sawtooth-shaped annular grooves in the surface of the back cavity. According to the bottom die body of the wheel die, temperature distribution of the whole bottom die body tends to be balanced and stable to a greater extent, and smooth and accurate formation of wheel castings is guaranteed.