From June to August in 2014, the distributions of chlorophyll-a (Chl-a) and correlations with environmental factors were investigated, the eutrophication status and causes were discussed in Tianchi Lake of Tianshan Mountains. The results showed that the Chl-a concentrations ranged from 2. 11 to 4. 06 µg. L -1 with a mean value of (2. 8 ± 0. 69) µg . L-1 in summer, the vertical distribution of Chl-a in different monitoring zones had a similar characteristic that the Chl-a concentration in epilimnionis (2-12 m) was higher than those at the surface (0-2 m) and hypolimnion (below 12 m). The Chl-a concentration had significant negative correlations with depth and conductivity, significant positive correlations with water temperature, pH value, the concentration of dissolved oxygen, the cell density of algae and TP, and less correlation with TN. The mean concentrations of TN and TP in Tianchi Lake in summer were 0.27 mg . L-1 and 0.035 mg. L-1 respectively, both of which exceeded the international standards of general eutrophic lakes. The modified Carlson trophic state index was used to assess the eutrophication status of Tianchi Lake, which is currently at a moderate eutrophication level. The reasons might be the increased nutrients attributed to soil erosion, and algae blooms due to the changes in the composition of hydrobios. Therefore, it is important to use ecological restoration and management to protect the water quality and surrounding vegetation, which may mitigate the risk of eutrophication in Tianchi Lake.