GB/T 7488-1987 is a water quality testing standard. The five-day biochemical oxygen demand (BOD5) is determined by dilute and inoculation methods. BOD5 reflects the content of biodegradable organic matter in water and is an important indicator to measure the degree of water pollution. The standard details the process from water sample dilution, microbial inoculation, to the determination of dissolved oxygen before and after five days of foster and the calculation of BOD5. The method is rigorous and suitable for various types of water samples. It provides key data support for assessing the pollution status of water bodies by organic matter and formulating water pollution control plans, and helps water quality monitoring and environment protection.