A square solid-state capacitor comprises a square aluminum shell, a capacitor core pack, a rubber plug and an aluminum shell cover plate, wherein the capacitor core pack is of a flat oval shape, the rubber plug and the aluminum shell cover plate are used for sealing the core pack in the square aluminum shell, the capacitor core pack comprises a positive electrode foil, a negative electrode foil, a piece of electrolytic paper, a positive lead and a negative lead, the electrolytic paper, the positive lead and the negative lead are arranged between the positive electrode foil and the negative electrode foil, a monomer and an oxidizing agent are immersed in the electrolytic paper, a conductive polymer is generated on the electrolytic paper from the monomer and the oxidizing agent, the monomer is one or more EDOT and derivatives thereof, pyrrole and aniline, and the oxidizing agent is one or more of iron p-toluenesulfonate, ammonium persulfate, sodium persulfate, ammonium sulfate and ferric chloride. The solid-state capacitor with a square structure is small in thickness (width) and long in length, the volume, capacity and density requirements of the solid-state capacitor are ensured, meanwhile, the demand for placing the solid-state capacitor in a flat space is met, and the space utilization ratio and the cooling performance of the capacitor are improved.