Abstract: Nylon 66 salt is a monomer of Nylon 66 resin. In order to ensure that adipic acid and hexamethylene diamine react in an equimolar ratio in the industry, adipic acid and hexamethylene diamine are generally made into Nylon 66 salt before the polycondensation reaction to produce nylon 66 resin. The article describes the technical application of the critical angle refractometer for sugar particle size measurement in the production of Nylon 66 brine solution and the operation and maintenance of the instrument.