The invention discloses an isobaric air chamber of a medium speed mill. The isobaric air chamber comprises an arc-shaped flow equalizing part mounted on the inner side of an outer side wall of the air chamber, wherein the thickness of the flow equalizing part is changed in the circumference direction of the outer side wall of the air chamber; the closer the arc-shaped flow equalizing part is to the position of an air inlet of the shell of the air chamber in the circumference direction, the smaller the thickness is. The arc-shaped flow equalizing part is a cone in the position close to a abrasive disk in the axis direction, and the closer the arc-shaped flow equalizing part is to the abrasive disk, the smaller the thickness is; the arc-shaped flow equalizing part is a column in the position away from the abrasive disk. The isobaric design inside the air chamber is achieved mainly because the arc-shaped flow equalizing part is welded on the inner side of the outer side wall of the air chamber and the width (or thickness) of the flow equalizing part is changed along the circumference.