Effects of harmful algal bloom (HAB) on bio-optical properties of coastal waters were studied. Bio-optical data were collected from 11 stations in the Dalian Bay, for an analysis of variable characteristics of biological factors, reflectance and absorption spectra as responses to HAB. The results indicated that, (1) the HAB which occurred in the Dalian Bay was caused by picophytoplankton. (2) Remote sensing reflectance showed an obvious difference with the normal waters: the strong absorption of the high concentration chlorophyll-a leaded to two reflection dips near 440 and 632 nm bands, a much larger fluorescence peak height around the red band accompanied by a "red shift", a sharp peak of oxygen response at 760 nm, and an enhanced reflection peak of suspended matter in the near-infrared band. (3) In addition, effects of HAB on absorption coefficient spectrum mainly reflected in the numerical size and much stronger absorption of ocean color constituents than the normal waters had been found, the a(ph) (440), a(ph) (675), a(d) (440) and a(g) (440) increased to 13.4, 14.5, 5.0 and 3.8 times of the values of normal waters, respectively. Variation features of bio-optical elements were examined and identified when HAB occurred in the Dalian Bay, which provides a way to monitor HAB by satellite remote sensing.