Abstract: Weeds are an important component in the farm land where a complex ecological relationship exists among weeds. crops and the environment. The investigation of this relationship has a great significance both in weed control for crop production and in theoretical study of cultivated vegetation.By using cluster analysis, an obvious correlation between weed types and soil types (correspond to soil genera) is found in alluvial plain where no obvious climate difference occurs. In the classification of weed types on dry farm land, the diversity of ecological conditions is of greater significance than dominant crops. Since most of the weeds have widely ecological amplitudes, the cluster analysis method is more objective than using dominants alone in weed classification. This paper also points out the worst weed species With their distribution, seasonal harmfulness and the degree of harmfulness in different environmental conditions.