Abstract:This paper deals with the variation law of photosynthetic rates of Eupatorium adenophorum Spreng and the relation between the variation law and I main enviromental factors. The difference of its photosynthetic rates was compared , with some farm crops. The main results are as follows:1. E. adenophorum is a herbaceous plant of C3 type and sun inclining to shade. The light saturation point is about 40000 Ix. The compensation point is about 700 Ix. The CO2 compensation point is about 80 ppm.2. The maximum net photosynthetic rate of E. adenophorum is about 23 CO2 mg.dm^-2.h-1. The diurnal variation law of its photosynthetic rates exhibits a curve with double peak (main peak at about 10 : 00 and secondary peak at about 16: 00). Its photosynthetic rate keeps higher level for a long time in a year.3. The change in the soil moisture content affects leaf‘s photosynthetic rate obviously. The photosynthetic rate of E. adenophorum grown in vegetable soil approximates to zero when the soil moisture content is reduced to 17, and what is more, the photosynthetic rate of the plant undergone drought tieatment on the third day after lewatering is 53 as much as the normal ones.Accordig to the above experimental results and climate characteristics which the dry and rain season aie very distinct in the harmed area, it seems that the dry season (from October to next February) is better time for controlling E. adenophorum.