[Objective] Through the discussion of diurnal changes of photosynthesis of Flaveria bidentis at the seedling stage under different soil moisture conditions,photosynthetic physiological basis of Flaveria bidentis to adjust to drought stress is investigated in order to provide a theoretical basis for prevention of further invasion. [Method] The seeds of Flaveria bidentis were planted in plastic buckets laid within all-transparent canopy. It was set with 3 soil moisture treatments,that is,the soil relative water contents （SRWC） were 80% （CK）,60% （T1）,30% （T2） of field maximum moisture capacity. Photosynthetic parameters and diurnal changes of relevant ecological factors of Flaveria bidentis at the seedling stage were measured and the differences between different treatments were compared. [Result] Under CK treatment,the diurnal changing curve of Pn of Flaveria bidentis was basically in a ＂bimodal＂ shape,while under T1 and T2 treatment,Pn was a ＂unimodal＂ curve. At about 11：30 am,CK reaches its first peak value of 31.83 μmol/（m2·s） and its second peak value of 23.34 μmol/（m2·s） appears at 15：30. At 13：30 T1 and T2 have their peak values. The diurnal changes of Tr of Flaveria bidentis leaves in the three treatments are all in a unimodal curve. There are no significant differences between CK and T2 treatments,while the diurnal water use efficiency in T1 treatment is obviously below the ones in CK and T2 treatments. [Conclusion] Drought stress reduces Pn,Tr,Sc and other physiological indices of Flaveria bidentis. The reason why Pn is reduced is due to both stoma and non-stoma factors. Drought stress also postpones the appearance of Pn peak value. Proper drought can efficiently promote water use efficiency of Flaveria bidentis and cause its anti-drought reaction.
