The invention discloses repairing process for a surface defect of magnesium alloy casting. The process includes steps of cleaning and machining the defect, cleaning welding wires, pre-heating, repairing welding, preserving heat and the like. The process has the advantage that by means of the pre-heating, fast welding, heat preservation and the like, a thermal stress range is small, magnesium casting is basically not deformed, a base material texture of repaired magnesium casting basically keeps original texture and performance, and a problem of conversion of metal magnesium casting texture in thermal cycle of welding repairing and a problem that a welding repairing area is cooled fast and uneven in contraction are solved. The repaired metal magnesium casting base material texture basically keeps the original texture and performance, cracks and a new welding regenerated defect are prevented, and welding deformation is effectively controlled. The repairing process is applicable to repairing of the surface defect of the magnesium casting and the repairing of surface allowance insufficiency, widens an application range, and has the advantages of being simple to operate, low in cost, easy to master and high in welding repairing efficiency.