The invention discloses a mesenchymal stem cell injection, a preparation method and application of the mesenchymal stem cell injection in preparing a medicine for treating diabetes; the mesenchymal stem cells are derived from human umbilical cord and placenta; the mesenchymal stem cell injection consists of components: human mesenchymal stem cells, human albumin, low molecular heparin calcium, compound amino acid, vitamin C of 0.5% and dissolving medium; and the dissolving medium can be a compound electrolyte solution or glucose or normal saline. According to the mesenchymal stem cell injection, the preparation method and the application of the mesenchymal stem cell injection in preparing the medicine for treating the diabetes, the mesenchymal stem cell injection is used for repairing injured islet beta cells, and the blood sugar is reduced by secretion of the endogenous insulin, therefore, a purpose of foundational treating the diabetes is achieved. A disease course of the diabetes can be reversed, patients are helped in escaping out of inconvenience and toxic and side effects of taking the endogenous drug and injecting the insulin as well as serious complications caused by poor control of the blood sugar; and the 1-type and 2-type diabetes are treated thoroughly.