Chlorophyll a mass concentration will increase when red tide breaks out. It causes the changes of waters inherent optical properties (IOP), such as phytoplankton absorption coefficients and beam total absorption coefficients, which lead to the changes of waters apparent optical properties (AOP). Optical buoy can realize continual observation of apparent optical properties in situ, so it is hopeful to monitor red tide by developing corresponding method based on optical buoy′s data. A semi-analytical monitoring method is developed for red tide by using optical buoy′s data obtained during the process of red tide. First, beam total absorption coefficients are retrieval from diffuse attenuation coefficient and remote sensing reflectance, which are calculated from optical buoy′s data, and combined with experiential underwater downwelling average cosine. Then phytoplankton absorption coefficients and chlorophyll a mass concentration are retrieved. The median relative errors of beam absorption coefficients, phytoplankton absorption coefficients and chlorophyll a mass concentrations estimated by the semi-analytical method are 8.6%, 34.9% and 38.9%, respectively. Compare with quasi-analytical algorithm (QAA) and statistical method, the advantages are that the retrieved results are better and all experiential parameters are from radiative transfer calculation, which don′t depend on buoy′s data and have limited effect on the results.