NOVELTY - The method involves reacting hydrocyanic acid as initial raw material with formaldehyde to generate hydroxy-acetonitrile, reacting hydroxy-acetonitrile in an aqueous ammonia-carbon dioxide system to obtain an aqueous solution of hydantoic acid, carrying out acid hydrolysis to obtain hydantoin crystals, purifying to obtain a highly pure hydantoin product, and performing synthesis reaction of hydantoic acid with hydroxy-acetonitrile in aqueous ammonia-carbon dioxide system using a combined-type continuous reactor.    USE - Continuous synthesis of hydantoin (claimed).    ADVANTAGE - The combined-type continuous reactor is adopted to synthesize hydantoin, so the device investment is effectively reduced, and the investment efficiency is improved. The method is automatically and conveniently controlled, and the continuity and the stability of production are guaranteed. The labor cost is saved, and the safe production operation of the whole process is guaranteed. The generation of harmful impurities is inhibited. The decomposition and polymerization of hydroxy-acetonitrile and other side reactions are effectively inhibited. The purity and the yield of the obtained product are improved. The method has lower requirements of raw materials, and fully and comprehensively utilizes resources. The production method is clean and environmentally-friendly.    DETAILED DESCRIPTION - Production method for continuously synthesizing hydantoin, involves reacting hydrocyanic acid as initial raw material with formaldehyde to generate hydroxy-acetonitrile, reacting hydroxy-acetonitrile in an aqueous ammonia-carbon dioxide system to obtain an aqueous solution of hydantoic acid, carrying out acid hydrolysis to obtain hydantoin crystals, purifying to obtain a highly pure hydantoin product, and performing synthesis reaction of hydantoic acid with hydroxy-acetonitrile in the aqueous ammonia-carbon dioxide system using a combined-type continuous reactor which is composed of a tubular reactor and a column bed reactor, where the tubular reactor is one portion of the whole reactor, one or several column bed reactors are connected in series as other portion of whole reactor, the two portions are connected in series to form a combined-type continuous reactor, entering the mixed reaction material from the tubular portion of combined-type continuous reactor to the column bed type portion, passing the reaction material through the column bed type portion of the reactor and flowing out from an outlet at the end of the reactor.