The invention provides a pretreatment method for repair welding of an austenitic stainless steel cast nuclear pump case. The pretreatment method comprises the following specific steps of: defect determining, defect coarse grinding, nondestructive flaw detection, symmetrical grinding, to-be-welded region accurate grinding and to-be-welded region cleaning, wherein during symmetrical grinding, positions which are symmetrical with defects with sizes greater than equal to 15 cm are ground at the same degree after the defects are completely ground off, so that welding slots which are symmetrical with the defects are formed; during to-be-welded region accurate grinding, front surfaces of the accurately ground welding slots are oval surfaces, and portions, which are 30 mm or less outside the borders, of the welding slots, are accurately ground; and during to-be-welded region cleaning, residual impurities and greasy dirt, which are 30 mm or less outside a to-be-welded region and the borders of the welding slots, caused by grinding and nondestructive flaw detection are eliminated. The method provided by the invention ensures that welding stress distribution is uniform after repair welding, and symmetrical participating stress can limit occurrence of deformation; and moreover, fewer impurity elements are introduced, and a repair welding region is free of welding defects, so that problems in repair welding are improved, occurrence of secondary defects is avoided, and the repair welding success rate of the austenitic stainless steel cast nuclear pump case is increased.