Solidago canadensis L. was treated with metsulfuron-methyl, fluroxypyr and iso-propyl glyphosate. The photosynthetic rate and chlorophyll fluorescence parameters were measured. The results showed that after treated 13 days later, the intrinsic conversion efficiency of light energy decreased, treatment A1B3 had the largest decline, which was 81.6 % of the control, the changes of treatments treated with iso-propyl glyphosate were not obvious; the photochemical quenching parameter of all treatments decreased, treatment A2B2 had the largest decline, which was 42.6 % of control; the photosynthetic electron transport rate decreased obviously, treatment A1B2，A1B3 and A2B2 had the largest decline, which was 20.0 % of control; the net photosynthetic rate decreased greatly, treatment A2B2 and A2B3 drooped more than others, which were 11.3% and 17.8% of control respectively. After treated 50 days later, the plants treated with metsulfuron-methyl and fluroxypyr were dead, whose net photosynthetic rates were zero. The net photosynthetic rates of the plants treated with iso-propyl glyphosate decreased to varying degrees, but plants were alive. Result shows that metsulfuron-methyl and fluroxypyr could be used to kill the Solidago canadensis L., the plants would be dead after treated 50 days later.