The invention provides an extrusion forming method of a magnesium alloy ring. The extrusion forming method comprises the following steps of blanking to prepare a magnesium alloy blank; sleeving the magnesium alloy blank on the outer wall surface of an inner concave die and positioning the magnesium alloy blank in a cavity; enabling an upper die assembly to press down the magnesium alloy blank on the outer wall surface of the inner concave die through a press so that the magnesium alloy blank slides downwards along the outer wall surface of the inner concave die; extruding and forming the magnesium alloy blank by a curved generatrix of the outer wall surface of the inner concave die; and ejecting the formed workpiece through an ejecting mechanism at the bottom of the cavity to finish demoulding. According to the forming method, near isothermal forming can be realized, material cracking is avoided, the forming yield of the large-size magnesium alloy ring piece is greatly improved, the manufacturing process of the large-size magnesium alloy ring piece is shortened, the production cost is reduced, the die is simple, the operation is convenient, the extrusion forming process can be greatly shortened, the formability of the magnesium alloy is effectively improved, and the large-degree reaming deformation is convenient.