Pearl River Estuary is an important region for South China

including Hong Kong and Macao. It has experienced tremendous environmental changes due to the fast social and economic development in Pearl River delta

especially in recent decade. It is rather difficult to realize the changes of water quality for this large complex estuary system over a long period by the traditional measurements. Remote sensing technology is used to monitor the evolution of marine water quality. The historic Landsat data over twenty-five years are applied to extract the changing information of ocean color. The results show an alarming fast trend of high turbid waters in Pearl River Estuary and Hong Kong coastal waters in these years. The algorithm of "Gradient Transition" is used to calibrate the resulting images to give quantitative analysis of turbid waters. Reasons for such deterioration are discussed in detail. A comparson method is proposed to process and analyze Landsat images to get feature of ocean color of Case 2 waters. The concept of multiple scatters is suggested for the study of turbid and shoal Case 2 waters near coastal zone.