Eutrophication has been one of threats to the aquatic environment nowadays. Learning the water quality and analyzing the possibility of eutrophication of Weiming Lake, an irreplaceable symbol of Peking University, is the scientific basis to improve the water quality and a reference to the protection of other artificial lakes. Hardly disturbed by anthropogenic activities, the water quality of Weiming Lake was good in nitrogen related indexes, but the indexes of COD, TP, and DO were at or even worse than Level V water quality in all the regions and seasons. The average TLI indexes of Weiming Lake was 56.1, slightly eutrophication. PCR-DGGE analysis showed the diversity of sediment bacterial community varied through seasons, higher in spring and summer, highly related with the location and the weight of nutrients in summer and autumn. Linear regression showed that the activities of algae may influence the sediment bacterial community through nitrogen fixation.