The utility model provides a pneumatic four-way seawater ball valve which comprises a valve body, four auxiliary valve bodies, a valve deck, sealing valve seat assemblies, a pneumatic actuation mechanism, a valve element and a valve rod, wherein the valve element and the valve rod are arranged in the valve body and the valve deck. The four auxiliary valve bodies are symmetrically arranged on the outer side of the valve body, the sealing valve seat assemblies are installed in annular grooves in the auxiliary valve bodies, the spherical sealing faces of the sealing valve seat assemblies and the spherical face of the valve element are tightly attached and sealed, and the pneumatic actuation mechanism is connected with the valve rod through a support. The ball valve is compact in structure and has good sealing performance and reliable corrosion resistance.