The invention provides a novel method for preparing a latent nonionic self-emulsifying epoxy curing agent. The method comprises the following steps of: reacting long alkyl chain fatty amine with diglycidyl ether to prepare an addition product of which the middle part is connected with a long hydrophobic side chain and two ends are provided with epoxy groups; and reacting polyamine with a carbonyl compound to prepare an imine compound which contains secondary amine, and sealing the end to prepare the latent nonionic self-emulsifying epoxy curing agent. Under water environment, the imine structure in the curing agent hydrolyzes hydrophilic amino, and the curing agent contains the long hydrophobic alkyl chain, so the curing agent has the functions of emulsifying and curing an epoxy resin. The curing agent can be used for water-based epoxy emulsion coatings, water-based epoxy resin modified emulsified asphalt and cement and the like, and has excellent adhesion performance of ketimine on a wet surface. Due to the introduction of hydrophobic chains, the stability of the imine curing agent is improved in the water environment, and the performance of the curing agent in the aspect of wet surface or underwater adhesion can also be improved.