The Qingcaosha reservoir at the Yangtze River estuary is an important source of drinking water, and its water quality must be assessed. This paper takes suspended solids in the water body as a key parameter to evaluate water quality and conducts related research using high spatial resources data of Landsat-8 Operation Land Imager (OLI) to objectively and scientifically evaluate the water quality of Qingcaosha reservoir. An inversion algorithm for suspended particulate matter (SPM) in Qingcaosha reservoir was developed using satellite data and field observations. Field observations were used to verify the accuracy of the developed algorithm. The results showed that the correlation coefficient was high and the root mean square error was low, indicating that the algorithm is effective. In addition, the inversion accuracy is high and the inversion result is credible. The Landsat-8 OLI data from 2013 to 2019 were processed using the proposed algorithm to obtain the average spatial distribution and time distribution characteristics of SPM. The SPM concentration in Qingcaosha reservoir was found to be the lowest in the estuary. The difference of SPM concentration in the reservoir’s upper part is the largest in winter and the smallest in spring. The concentration of SPM in the reservoir’s middle and lower parts is low, and the change is small. According to the research results, the Qingcaosha reservoir at the Yangtze River estuary is clean and has good water quality.