To test the effect of adjuvant on the efficacy of herbicides in a fast and non-destructive way is very helpful for selecting a right spray adjuvant for herbicide, which is an important strategy to enhance the efficacy of herbicides, reduce application dose, and enhance environmental safety. Experiments were conducted in the laboratory and greenhouse to study the effect of 2 adjuvants - Methylated seed oil (MSO) and organosilicone on the efficacy of topramezone on grass weed giant foxtail (Setaria faberi Herrm.) and broadleaved weed velvetleaf (Abutilon theophrasti Medik.) using weed leaves chlorophyll fluorescence measurement and whole plant biomass test. The results indicated that the top leaf maximum quantum efficiency (Fv/Fm) of two weeds treated by herbicide mixed with MSO adjuvant was significantly lower than that of treated by herbicide alone from the 2–3 days after treatment, while the difference between treatments of herbicide mixed with organosilicone adjuvant and herbicide applied alone was not significant. Results of the whole-plant pot tests showed biomass of the treatment of topramezone mixed with MSO was significantly lower than that of herbicide applied alone. This is similar to the result of chlorophyll fluorescence test. Chlorophyll fluorescence measurement has proven to be an attractive tool for studying the effect of the adjuvants on the efficacy of herbicide.