The invention discloses a gating system of a large-section iron casting for an medium pressure external cylinder in a steam turbine; the gating system comprises a weighing ladle, a sprue, a runner and an ingate, the runner consists of an upper runner and a lower runner, a refractory ceramic filter is arranged between the upper runner and the lower runner, the minimum sectional area ratio of the sprue, the runner, a transition sprue, a transition runner and the ingate is 1.2:1.34:1:1.07:1.24; the sprue, the upper runner and the lower runner are arranged at the middle part of the casting, and the ingate of an ingate is arranged at the inner wall of an axle hole of the casting; and the gating system of the structure mainly plays a role in buffering, decompressing and steady entry of melted iron, the molding sand in the scour cast is avoided, the casting is heated evenly, with the advantages of being not easy to be mingled with impurities, carry sand, produce air gaps and cracks.