An analysis of the case of aircraft-icing occurred in Taiyuan airport of Shanxi Province 21 December, 2004 is made. It shows that the upper-trough in Hetao area and the high-pressure over the North China composed the background of this event. The equivalent temperature of environment lied between -6℃ and-10℃ was necessary for severe icing when the environment had plenty of moisture. The weak ascend velocity and cold advection are favourable to ice accumulation; WAFS NWP products can effectively describe the dynamics structure of synoptic system. A new icing index can better predict the icing event than the recommended icing-index by ICAO.