Abstract:The spatio-temporal variation of the water quality in Hongze Lake were studied with the least favorable value method and the anomaly coefficient method based on the monthly monitoring data from 2010 to 2020. The results show that TN, CODMn, and TP are the most important factors that affect the water environment of Hongze Lake. With regard to the temporal distribution, the monthly TN concentration in Hongze Lake decreased continuously, the monthly CODMn concentration gradually stabilized, and the monthly TP concentration increased first and then decreased. The annual TN concentration decreased significantly, while the annual TP concentration decreased slightly from 2010 to 2020. The least favorable value of TN showed a downward trend, with the dry season having a significantly greater effect on the increase of TN than the flood season. The least favorable value of TP showed a slight upward trend, which mainly occurred in August, September, and October, with a probability up to 81.8%. With regard to the spatial distribution, the TN and CODMn anomaly coefficients in the lake area in Huai’an were mostly positive, while the coefficients in the lake area in Suqian were mostly negative. The TP anomaly coefficient was mostly negative in the lake area in Huai'an, while mostly positive in the lake area in Suqian. Currently, control of TN and reduction of the total emissions of CODMn are the two prominent issues needed to be addressed in the Huai'an area, and TP control is an urgent issue to be strengthened in the Suqian area.