The single and combined effects of Cd2+ and S-metolachlor on the chlorophyll content and chlorophyll fluorescence parameters of Scenedesmus obliquus were studied by using standard toxic testing method. Both Cd2+ and S-metolachlor had effects on the chlorophyll content and fluorescence parameters such as F0, FV/Fm, FV/F0, Y( II), qP, NPQ and rETR after 96 h-exposure, showing that Cd2+ and S-metolachlor damaged the PS II in algae, inhibited the primary reaction of photosynthesis, stopped the process of photosynthetic electron transport, and destroyed its ability of heat dissipation. The effects of Cd2 + on the chlorophyll content and fluorescence parameters were greater than those of S-metolachlor, and the effects increased with the increasing concentration. The average drop of Y( II ) was 62. 5% in the control group when the light intensity was 231 μmol (m2.s) -1 , and it was 68. 0% , 82. 5% and 100% respectively in Cd2+ -treated groups, and 66. 1% , 72. 1% and 79.6% respectively in S-metolachlor-treated group with the increasing concentration. The combined effects of Cd2+ and S-metolachlor on the chlorophyll fluorescence parameters were mainly due to the impacts of Cd2+.