The pollution of lakes in water valley is particular because of the influence of the characteristic of upriver contamination. Taking Baiyangdian，which is a typical shallow weedy lake in the north of China, as a research object，combining the measured water quality data with the spatial data，using the method of Principal Components Analysis based on factor analysis，the water quality parameters of Baiyangdian are summarized into five principal components, namely the index of eutrophication, the index of organic pollution, the index of acid and alkali，the index of algae quantity and the index of plant growing environment. The meaning of each principal component and the pollution mechanism of Baiyangdian are analyzed in this paper to provide scientific basis for the reasonable utilization and the pollution control of Baiyangdian.