Wetland plays an irreplaceable role in many aspects and waters are important part of wetland, the water quality can easily reflect the situation of Wetland. In this study, water quality was assessed on the basis of 5 parameters (DO, NH 4 + -N, TN, TP and CODcr) that were monitored monthly at 5 sites (N 1 ,N 2 ,N 3 ,N 4 and N 5 )from April, 2014 to March, 2015 of the Nanhai Lake in Baotou, China by water pollution index method and comprehensive water quality identification index method. The twelve monitoring months were divided into wet season (Mar., Aug. and Sep.), normal season (Jan., Feb., Apr., Nov. and Dec.) and dry season (May., Jun. and Jul.). The assessment results determined using the water pollution index method showed that the water quality of all the five monitoring sites were inferior Ⅴ, the main contamination was COD. The comprehensive water quality identification index showed that the water quality of the Nanhai Lake were classesⅤ, except for the N 2 in wet season and dry season, the N 1 in dry season and the N 5 in normal season, which were classes inferiorⅤ. All the five monitoring sites don’t achieving the desired water quality standard. According to the analysis, domestic discharge, industrial activities and developed recreation were major threats to water quality of Nanhai Lake.