The invention discloses a sectional type crystallization device for magnesium smelting. The sectional type crystallization device comprises a reaction tank. A crystallization cylinder is coaxially installed on the top of the reaction tank. A cooling water jacket is coaxially arranged around the crystallization cylinder in the circumferential direction. A water inlet pipe is arranged at the bottom of the cooling water jacket. A water outlet pipe is arranged on the top of the cooling water jacket. The inner diameter of the cooling water jacket is larger than the outer diameter of the crystallization cylinder. The crystallization cylinder is composed of an upper crystallization cylinder body, a middle crystallization cylinder body and a lower crystallization cylinder body from top to bottom. The upper crystallization cylinder body, the middle crystallization cylinder body and the lower crystallization cylinder body communicate with one another. The height of the upper crystallization cylinder body and the height of the lower crystallization cylinder body are both smaller than the height of the middle crystallization cylinder body. The thermal radiation area between the upper crystallization cylinder body and the cooling water jacket and the thermal radiation area between the lower crystallization cylinder body and the cooling water jacket are both smaller than the thermal radiation area between the middle crystallization cylinder body and the cooling water jacket. Different crystallization temperatures in the upper crystallization cylinder body and the lower crystallization cylinder body are achieved, and therefore various metal impurities, such as iron, nickel, copper, potassium and sodium, and magnesium are independently crystallized out.