Abstract: In this paper,the chlorophyll a retrieval algorithm for the MODIS was developed based on the in-situ chlorophyll-a concentration and remote sensing reflectance spectra in the Bohai Sea from 2013 to 2021.Through the classical OC3 model,the in-situ data were used to fit and analysis to obtain the local parameters of the model suitable for Bohai Sea.The R²,MAPE and RMSE of the remote sensing inversion results were 0.84,24.77% and 5.56 μg/L,respectively,with good inversion accuracy.Using this algorithm,the monthly,seasonal and annual average chlorophyll a concentration in the Bohai Sea from 2002 to 2021 were retrieved,and the temporal and spatial variation characteristics were analyzed.Meanwhile,the environmental response was analyzed based on the proportion of the sea area with water quality not meeting Grade Ⅱ in the Bohai Sea from the beginning of this century.The results showed that from 2001 to 2021,the proportion of the sea area with water quality not meeting Grade Ⅱ in the Bohai Sea had the same trend with chlorophyll a concentration in the same period,showing an inverted "V" shape trend of first getting worse and then getting better.The average ratio of the proportion of the sea area with water quality not meeting Grade Ⅱ and chlorophyll a concentration in 5 years reflected the overall trend of water environment change in the Bohai Sea from 2001 to 2021,and chlorophyll a concentration had a faster response to the improvement of the Bohai Sea water environment.