The invention provides an ammonium chloride preparation system and an ammonium chloride preparation method for an ash and slag plasma melting treatment system. The ammonium chloride preparation systemcomprises an ammonia water spraying tower, a concentrator and a crystallizer purifying system which are sequentially connected, and a waste heat recoverer in the plasma melting treatment system provides steam for the concentrator and a crystallizer. The problems of secondary hazardous wastes, high energy consumption and complex devices in existing systems for absorbing acidic gases generated by the ash and slag plasma melting treatment system are solved in the invention.