Disclosed is an apparatus and method for preventing deformation of a plastic battery pack case for a vehicle, in which the side of the battery pack case is subjected to reverse deformation to absorb the deformation occurring after compression molding and to ensure the dimensions of a mounting portion of various components including battery packs, thereby ensuring an internal space of the battery pack case. To this end, the present invention provides an apparatus for preventing deformation of a plastic battery pack case for a vehicle, the apparatus including: a base having a case insertion space; a fixing portion for fixing a battery pack case inserted into the case insertion space; and a deformation producing portion inserted into the battery pack case and preventing the case from being deformed by pressing the inside of the case to be reversely deformed in the lateral direction.