The invention relates to a method for purifying long-carbon-chain cyanic acid products, belonging to the technical field of separation. The method comprises the following steps: mixing ammoniated products and water, then regulating pH value by using alkali, fully stirring, and standing for layering to obtain an upper oil layer as a component A, namely NC-R-CN; continuously dropwise adding dilute-acid solution into the remaining lower-layer aqueous solution, regulating PH of the solution, fully stirring, standing for layering, and collecting an upper-layer oil phase which is a component B, namely HOOC-R-CN; continuously dropwise adding dilute acid into the remaining lower-layer aqueous phase, regulating PH, fully stirring to fully precipitate all solids, cooling, and carrying out centrifugal separation to obtain a component C, namely HOOC-R-COOH. According to the separation method provided by the invention, long-carbon-chain cyanic acid product can be separated, especially 8-cyanooctanoic acid and 9-cyanononyl acid which are front intermediates of nylon 9 and nylon 10; and According to the method, the process is simple and reliable the repeatability is strong and the method is applicable to large-scale industrial application.