A sealing cap comprises a flange plate and a flow blocking ring, wherein the flange plate is of a basin-shaped structure provided with a through hole and defined by a pair of flange assemblies; each flange assembly comprises a flange plate base and a clamping ring; each flange plate base is a half-arc groove body; the upper portion of each half-arc groove body is provided with a hanging lug for fixing the sealing cap to a flow guiding cover wall plate; the middle of the lower portion of the half-arc groove body is provided with a semicircular hole used for being assembled into the through hole; each clamping ring is arranged on the corresponding half-arc groove body; and a gap for installing the flow blocking ring is reserved between each half-arc groove body and the corresponding clampingring so as to guarantee that the flow blocking ring can flexibly move in the flange plate. The sealing cap is installed on an airplane aircraft gun flow guiding device, the gap between a gun barrel and the flow guiding cover wall plate can be effectively reduced, and therefore gas flow discharged backwards by the flow guiding device can be controlled; and meanwhile, the flow guiding and sealing work of the aircraft gun flow guiding device is facilitated.