Seasonal dynamics and spatial distributions of chlorophyll-a concentration and its relationship to environmental factors in Lake Erhai from May 2010 to April 2011 were investigated. Results showed that the seasonal dynamics of chlorophyll-a concentration in Lake Erhai were obvious, ranging from 4.11 to 24.30 μg/L and with the annual mean value 10.4 ± 6.5 μg/L. The minimum and maximum of chlorophyll-a concentration appeared in March and September, respectively. The chlorophyll-a concentrations were much higher in summer and autumn while relatively lower in winter and spring. Spatially, chlorophyll-a concentrations were the highest in the southern zones of the lake, followed by the northern and the middle zones. Pearson correlations and principal component analysis suggested that the chlorophyll-a concentration has negative correlation with transparency in different zones of Lake Erhai, while has positive correlation with water temperature. The chlorophyll-a concentration was correlated significantly with total nitrogen in the northern and southern zones of Lake Erhai, while correlated with total phosphorus in southern zones. Based on the modified Carlson trophic state index, the comprehensive TSI was 50.6 in Lake Erhai, indicating a mesotrophic state.