The invention discloses a preparation method of copper glycinate. The preparation method comprises the following steps: (1) dissolving copper oxide: dissolving the copper oxide into ammonia water to form a copper-ammonia complex; (2) carrying out a synthetic reaction: adding glycine and the copper-ammonia complex into a reaction kettle according to a certain ratio and reacting; heating and reacting for a period of time to obtain reaction liquid; (3) purifying a product: cooling the reaction liquid and centrifuging to obtain a copper glycinate solid 1 and filtrate; decompressing and distilling the filtrate to obtain a copper glycinate solid 2 and a liquid fraction; drying the copper glycinate solid 1 and the copper glycinate solid 2 to obtain a product; (4) recycling ammonia: absorbing ammonia gas generated in a reaction process by an absorbent, so as to obtain recycling liquid; using the recycling liquid to the step (1) to replace ammonia water for dissolving copper oxide. The invention provides the preparation method of the copper glycinate, which can avoid interference of foreign ions and can also improve the reaction speed.