The invention relates to a closed diphenyl-methane-diisocyanate curing agent and a preparation method thereof, and the curing agent is generated from MDI, a self made molecular weight regulator, a micromolecular triol, a blocking agent and a stabilizing agent in an organic solvent by reaction. The preparation method comprises two steps: firstly, the molecular weight regulator is prepared by reaction of MDI and the blocking agent; secondly, MDI is prepolymerized with triol under the effect of the self made molecular weight regulator, and then is reacted with the blocking agent for preparing the closed curing agent, and finally the storage stabilizing agent is added. The invention is characterized in that: 1. the deblocking temperature of the closed curing agent is lower than that of the closed TDI curing agent; 2. the NCO percentage content of the curing agent is higher than that of the MDI prepolymer curing agent; 3. the curing agent has good storage stability, and is mainly used as a curing agent ingredient of the high temperature baking type polyurethane paint and the adhesive, and can be prepared to be solvent type and water soluble type closed curing agent.