Disclosed herein is a method of manufacturing an inner panel of a vehicle tailgate, which is formed in a bending plate shape and to which reinforcing members are coupled to improve rigidity, including: temporarily coupling a plurality of sub-blanks to a main blank by spot welding; forming the main blank into an inner panel and the plurality of sub-blanks into a plurality of reinforcing members, using a forming mold; and permanently coupling the plurality of reinforcing members to the inner panel.