This document (GB/T 21789-2025) specifies the method for determining the flash point of petroleum products, chemical reagents and Miscellaneous liquid materials using the Abel closed cup method. The standard details the key content such as instrumentation requirements, sample preparation, test procedures, result calculation and precision, and provides a standardized test basis for security evaluation, product quality control and transportation classification. This method is suitable for determining liquids with a flash point in the range of -30 ° C to 70 ° C.