The present invention relates to a non-peptidyl polymer-insulin multimer comprising two or more of a non-peptidyl polymer-insulin conjugate prepared by linking a non-peptidyl polymer and insulin via a covalent bond, in which the conjugates are complexed with cobalt ion to form a multimer, a method and kit for the preparation of the multimer, a pharmaceutical composition for the prevention or treatment of diabetes comprising the multimer as an active ingredient, and a method for preventing or treating diabetes by administering the composition to a subject.