A square billet crystallizer mechanism for vertical continuous casting of magnesium alloy, which belongs to the metallurgical field of metal materials, comprises a crystallizer main body, a frame and a water inlet. The square billet crystallizer is a cuboid shape, a coaxial cuboid hollow body with four round transition corners is processed inside the crystallizer, and a metal plating coat is plated inside the crystallizer. An O-shaped water channel consisting of horizontal water channels and vertical channels mutually intersecting and communicated with each other is processed on four walls inthe crystallizer, and secondary water outlets distributed uniformly and having a diameter gradually decreasing from the middle section to both sides is processed to keep communication with the lower water channels. The included angles between axis lines of all of the water outlets and the axis line of the crystallizer are identical to each other. The square billet crystallizer mechanism manufactured by low-cost common casting method has the advantages of simple structure, uniform cast billet cooling, high production efficiency, high quality of the cast billet and wide application, and is applicable for continuous casting of different materials such as steel, aluminum alloy, magnesium alloy, etc.