Abstract: The brown tide of picophytoplankton algal was a new marine ecological disaster in China.It occurred frequently in Qinhuangdao coastal sea.A method of brown tide detection using remote sensing technology was discussed.The abnormal ocean color was extracted by retrieving chlorophyll-a (Chl a) and sea surface temperature (SST) with MODIS data.On this basis, the brown tide information was finally extracted by comparing the color difference between brown tide water and non-brown tide water in the true color composite image of HJ-1.The brown tide of picophytoplankton algal occurred in Qinhuangdao coastal sea during the May, 2014 was analyzed at last.The results showed that the abnormal ocean color would be happened when Chl a≥2.5 mg/m³ and 15℃≤SST≤25℃.The color of brown tide water was yellow-green in the true color satellite image.Therefore, this method was effective for monitoring the brown tide of picophytoplankton algal.It lays a good foundation for the operational monitoring brown tide in the future.