The invention provides a staged fracturing method of a horizontal well. The method comprises the steps that a fixed base is arranged on the inner wall of a casing pipe in the center axis direction of the casing pipe according to the stage number of fractured intervals in the casing pipe; the casing pipe is put into a preset position inside the horizontal well; and the second stage to Nth stage of the fractured intervals are sequentially subjected to following operations, a soluble ball seat is arranged at the lower end of a perforation tool string, the perforation tool string and the soluble ball seat are pumped into the corresponding fractured interval, the soluble ball seat is controlled to form a closed soluble ball seat, the outer diameter of the closed soluble ball seat is greater than the drift diameter of the fixed base, the perforation tool string is controlled to conduct perforation in the first preset position in the corresponding fractured interval, the perforation tool string is lifted out, the corresponding fractured interval is subjected to fracturing treatment, and a soluble ball seat is thrown into a through hole of the closed soluble ball seat, so that the corresponding fractured interval is plugged.