The concentration of chlorophyll-a is a key indicator for evaluating the eutrophication of coastal water， and the research on the influencing factors of coastal chlorophyll-a is significant to marine environmental protection. However， the existing studies mostly focus on the effect of natural factors on the concentration of chlorophyll-a in coastal waters， ignoring the human activities. Therefore， this paper uses nighttime light brightness to characterize the intensity of human activities， and divides cities on the southeastern coast into three types based on its relationship with coastal chlorophyll-a concentration， and also combines sea surface temperature， wind speed， solar radiation， precipitation and human factors， using the Generalized Additive Model （GAM） to analyze the impact of multiple factors on coastal chlorophyll-a concentration in three types of cities in different seasons. The results show that the change of chlorophyll-a concentration was dominated by natural factors in type I cities such as Fuzhou and Shantou， the dominant factor in spring is wind speed， and the sea surface temperature in summer， autumn and winter； while human activities have little effect. Type II cities such as Zhuhai and Dongguan are dominated by natural factors. The dominant factor is wind speed in spring， autumn and winter， and the sea surface temperature in summer； human activities have a greater promoting effect in summer and autumn. Type III cities such as Shenzhen and Hongkong are dominated by human factors. Human activities have the greatest impact on chlorophyll-a concentration in spring， summer and autumn and they are a negatively correlated. In winter， sea surface temperature has the greatest impact.