The invention provides a method for forming medicament slow-release nanoparticles from inverse miniemulsion, and relates to the fields of polymerization synthesis and biomacromolecules, and the like. Chitosan is used as a base material, modified chitosan miniemulsion emulsion particles are formed in situ by the miniemulsion polymerization method, water-soluble medicament is added in order to prepare the water-soluble medicament slow release nanoparticles. The method is advantageous in that the modified chitosan is formed in situ and is used as a co-stablizer/stabilizing agent for polymerization of the miniemulsion, the particle size of the miniemulsion reaches 50 nanometers, and the particle size distribution is uniform; the method that the water-soluble medicament is dropwisely and slowly added is used in order to form a stable water-soluble slow release medicament miniemulsion; an oil and water azeotrope method is used in order to form the slow-release nanometer particles of the water-soluble medicament with good dispersibility.