Abstract: Two surveys were investigated in Qinzhou bay during April and September 2015 for collecting the surface seawater information on 18 environmental indicators including temperature, salinity, pH, dissolved oxygen, chemical oxygen demand, chlorophyll a, petroleum hydrocarbon, nutrients and heavy metal. Principal component analysis (PCA) was applied to analyze the collected data, and the potential driving variables effect the water quality were assessed. Four principal components were extracted from the 18 environmental variables, which explained 73.68% of the total variance in the original data set. The PCA indicated that pollution was more serious in the inner bay (Maowei sea) than that in the outer bay. The most serious pollution was in April, followed by September. River input, aquaculture, phytoplankton growth, and hydrodynamics might be the significant factors influencing the spatial-temporal variation of water quality in Qinzhou bay. The correlation analysis suggested that total nitrogen, salinity, pH, Cd and Zn were the main factors influencing the water quality. Terrestrial inputs and aquaculture activities were the main sources of the Maowei sea pollution. Management measures should be taken to protect environments along the Qinjiang River and Maolingjiang River and plan scientifically cultivation scale in order to improve the water quality in the Qinzhou bay.