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In order to effectively forecast or forewarn squall lines and verify the effectiveness of the nowcasting system, a squall line occurring on 1 August 2006 in Beijing is analyzed by using conventional weather data, automatic weather station data, Doppler radar data, and the nowcasting system. The results suggest that the squall line occurred under the favorable weather conditions. The severe storms on the squall line had large radar reflectivity and high tops of echoes. There was a divergence at lower level, a convergence at middle level, and a divergence at upper level in the velocity cross section of the squall. The VIL (Vertical Integrated Liquid water) of the storms increased rapidly to a large value, which is a good indictor of hailstorm. When the probability of severe hail is larger than 30% according to the hail index products, a hail storm is very likely to happen. The convergence line in the boundary can trigger and intensify convections. The nowcasting system is a good tool for nowcasting severe convections.