【Background】 Herbivore injury has a direct effect on the growth and performance of host plants through photosynthetic suppression. However, changes in the photosynthetic activity of host plants affected by mutualism between honeydew-producing hemipterans and tending ants remain poorly understood. 【Method】 The effects of interaction between an invasive mealybug Phenacoccus solenopsis and its tending ant Paratrechina longicornis on chlorophyll characteristics of infested cotton Gossypium hirsutum leaves were observed through a chlorophyll fluorescence measurement system under greenhouse conditions. 【Result】 P.longicornis numbers increased with P.solenopsis numbers. However, over time, plants infested with had a lower number of mealybugs than uninfested plants. Changes in light utilization efficiency were induced by P.solenopsis feeding of infested cotton leaves. After P.solenopsis feeding injury for 20 d, the light utilization efficiency compared with the control was reduced by 37.0% and 53.5% for without- and with-ant treatments, respectively. Changes in maximum relative electron transport rate were also induced by P.solenopsis feeding injury, and the influence was more obvious after 20 d or with tending ant infestation. However, the light saturation coefficient describing the resistant capacity of a sample to glare was not influenced by P.solenopsis with or without ants. 【Conclusion and significance】 The interactions between P.solenopsis and P.longicornis and their negative effects on the photosynthetic activity of cotton leaves could have been caused by P.solenopsis feeding rate by P.longicornis.