The invention provides a powerful-extrusion stable knife switch. The switch comprises a shell, a knife holder is designed inside the shell, and a knife is hinged inside the shell at the front end of the knife holder. Block pieces are designed at the two sides of electricity connection pieces used in pairs. Hydraulic cavities are designed below the electricity connection pieces of the knife holder. Vertically extending slide bars between the paired electricity connection pieces are designed in the hydraulic cavities respectively. The bottom ends of the slide bars extend into the hydraulic cavities respectively. The slide bars and the hydraulic cavities are assembled in a sliding seal way. An elastic capsule is clamped between each electricity connection piece and the corresponding block piece, and each elastic capsule is connected into the corresponding hydraulic cavity through a vertical tube. The hydraulic cavities are filled with insulating liquid. When the knife switch is closed, the electricity connection pieces are clamped to the middle, and the electricity connection performance is better. The longer the use time of the switch is, the better the electricity connection performance is. Because of greater clamping force, the knife is not easily opened by touch, and the switch is safer in use.