Lake water quality is closely related with human activities, and human activities are the driving factors in lake water quality aggravating. Analyzing the spatial-temporal trend of Lake Dianchi in recent ten years, taking proportion of interrupted landuse by human beings, population density, and GDP per square kilometer as human activity indicators, this paper investigated the relationships between lake water quality and human activities in Lake Dianchi watershed. Firstly, as Lake Dianchi is divided into two parts, Lake Caohai is greatly different from Lake Waihai in terms of water quality. Lake Caohai was polluted seriously and getting worse year by year, while the concentrations of NH₃-N, TN and TP in Lake Waihai were much lower than those in Lake Caohai, and changed slightly in past ten years. The concentration level of COD_Mn in Lake Waihai and its changing situation in past ten years was similar to those of Lake Caohai. Secondly, town construction landuse, population density, and GDP per square kilometer were leading factors in causing the differences between Lake Caohai and Lake Waihai. Meanwhile, city and town expanding, and population and GDP growing led to Lake Caohai polluted seriously, while Lake Waihai was less polluted or sometimes, even better (such as TP), perhaps because of non-point pollution control from agriculture and rural area, or the pollution-cutting-engineering at some inflow rivers' watershed.