According to the relationship between conventional observational data, Lband radar data, and Air Pollution Index (API), an index of meteorological conditions for pollutant diffusion (Index 〖WTBX〗A〖WTBZ〗) for Chongqing is established, which consists of three meteorological elements and can be used to determine the pollution potential through grading by diagnosing the climatic probability in the corresponding periods. The correlation analysis indicates that there is close relationship between API and meteorological data at 08:00, not at 20:00; among the surface meteorological factors, the correlation coefficient between API and the 24hour pressure variation at 08:00 is the largest, and the effect of temperature on API is different in various seasons; the wind velocity at 150 meters and the temperature difference between the ground and 300 meter height exhibit good relationship with API in the Lband radar data. Index 〖WTBX〗A 〖WTBZ〗is therefore composed of the 24hour pressure variation at 08:00, the wind velocity at 150 meters, and the temperature difference between the ground and 300meter height. The application proved that Index A is reasonable and performs well in the operational service.