Stable weather index (SWI) of Beijing is developed to meet the demand of quantitative description of the degree of stable weather which is conducive to the formation of fog and haze in China. During a heavily polluting process in January 2015, both the SWI and AQI (air quality index) showed consistent trend: stable growth in the developing stage and rapid decrease in the ending stage. SWI is also used to assess the effect of reduction measure of pollution source. Under the similar stable weather as described by SWI, reduction measures contained air pollution in Beijing during APEC. SWI is developed further based on meteorological data lasting 13 years. Subindex of any parameters is defined as the conditional probability of foghaze divided by the climatology probability when the threshold of the parameter is met. The top 10 stableweatherimplication parameters are chosen, of which subindexes are added up as the final SWI. The developed SWI shows better correlation with PM2.5 concentration which means better characterization of stable weather.