Abstract: The value of A is one of the key parameters in calculating environmentpurification capacity (EPC) by A-P value method. Proper determination of A value is favorable to protecting the air quality as well as the benefits of the companies and organizations emitting air pollutants. Based on the general principlesof the A value method in single-box model, H_i was ascertained by Technical guidelines for environmental impact assessment-Atmospheric environment (HJ/T2.2-93), and the single-box model method amended for calculating the value of A at various atmospheric stabilization levels. Based on the range of A in various regions advanced by Technical methods for making local emission standards of air pollutants(GB/T13201-91), the standard compliance rate method was advanced accordingto daily averaged standard compliance rates, and differentiated as the probabilistic formula method and chart method in accordance with different methods in ascertaining compliance rates. The explanations on how to use the methods were also provided. Taking a developing area in the Yangtze River Delta as example, the value of A was calculated by the three methods and the reliability demonstrationof the results taken. The character of each method as well as their applicationconditions was finally analyzed