The current method of runway width design is based on requirement of airplane, and don't consider how to reduce the blockage probability of runway when runway attacked by enemies. So the paper analyses the process of blockage runway by sub-munitions missile, and build model of blockage runway about runway width. Then through analysis the process and model，the major factors that will influence the blockage probability of runway is got. Based on relations between influence factors and blockage probability，the relationship between the blockage probability of runway and runway width is got under special condition。At last, the paper contrasts the economy of adding runway width and reducing the blockage probability of runway, then puts forward the concept applied defense factors in runway design