Abstract: Based on the in-situ remote sensing reflectance (R_rs) and chlorophyll a concentration (Chl a) data in the Qinhuangdao coastal area from 2013 to 2018, here a regional Chl a retrieval model was developed specifically for the Sentinel-2 MSI image. The results showed that the correlation coefficients between R_rs(443)/R_rs(560), R_rs(490)/R_rs(560) and Chl a were generally higher than that of other bands or combinations. The classical OC3Mv6 algorithm was used for fitting, the best Chl a retrieval model in the Qinhuangdao coastal area was obtained with R²=0.804, MAPE=40.2% and RMSE=4.73 mg/m³; using the in-situ Chl a data on July 6, 2016, the accuracy of the Sentinel-2 MSI remote sensing inversion result was tested, and the MAPE was 35.9%, which basically meet the application requirements; the regional Chl a retrieval model was carried out for the Sentinel-2 MSI image in February, May, July and October 2020, and it was found that the variations of Chl a were significant in spring and summer, while Chl a in autumn and winter were relatively uniform, and Chl a in coastal waters in spring and summer was significantly higher than that in autumn and winter.