An extruded hollow plate and an electric vehicle battery casing formed by combining the extruded hollow plates. The extruded hollow plate has a plate-shaped body having a constant cross-section, a cavity is provided inside the plate-shaped body, a protrusion and/or a groove is provided at an end of the body, the protrusion is bent downward, the groove opens upwards as a hook, and the arc surfaces forming the protrusion and the groove each comprise at least two involute surfaces. Compared with the existing battery box manufacturing process of friction stir welding, the combining and bonding connection manner has the significant advantages of rapid production speed, a low device cost, high flatness, etc.