The invention discloses a forming process of a medium-strong heat-resisting magnesium alloy triangular sectional material. The forming process comprises the following steps of smelting in a large furnace, semi-continuous casting, homogenizing, annealing, forging in different directions, extrusion molding and molding after-heat treatment. The forging in different directions is carried out on a hydraulic machine, a blank is thermally preserved for 2 to 4h at the temperature 370 to 400 DEG C before the forging and headed for 3 times. The extrusion molding is carried out on a horizontal extruder, the extrusion ratio is 10 to 14, and the length of the extruded triangular sectional material is more than or equal to 1000mm. The sectional material is high in strength and good in heat resistance, and the performance requirement of structural components in the aerospace field can be met. After being thermally treated, the yield strength of the sectional material in the extruding direction at the room temperature is more than or equal to 265MPa, the tensile strength is more than or equal to 375MPa, and the elongation rate is more than or equal to 9 percent.