The invention discloses a large-diameter moderately-strong heat-resisting magnesium alloy thick-wall barrel-shaped piece forming process. The process includes the steps of large-furnace smelting, semi-continuous casting, homogenizing annealing, varying-temperature multi-direction forging, mechanical punching, ring rolling forming and aging treatment. The varying-temperature multi-direction forging is conducted on an oil press, pressing speed is 200-400mm/min, upsetting reduction in pass is 30-40%, drawing down reduction in pass is 5-10%, blank before ring rolling forming is kept at the temperature of 380-400 DEG C for 4-8 hours, and rolling deformation is 60-80%. The thick-wall barrel-shaped piece 700-1100mm in outer diameter, 50-100mm in wall thickness and 300-700mm in height is manufactured. The thick-wall barrel-shaped piece is high in strength and good in heat resistance and meets size and performance requirements of structural parts in the aerospace field.