GB/T 13200-1991 is a water quality testing standard, focusing on the determination of turbidity. Turbidity reflects the scattering and absorption degree of suspended particles in water on rays of light, and is an intuitive indicator to measure water quality. The standard provides a variety of measurement methods, including scattering method, transmission method, etc., to clarify the principle of each method, instrumentation and equipment, operation steps and result calculation. From water sample collection, pretreatment to final measurement, the process is standardized. The standard is applicable to all kinds of water bodies, providing an important basis for water quality assessment, water treatment process optimization and environment monitoring, and helping to ensure a high-quality water environment.