Abstract: To investigate effects of nitrogen fertilizer on photosynthetic physiology of Solanum nigrum under cadmium stress, chlorophyll contents and chlorophyll fluorescence characteristics of S. nigrum under different nitrogen fertilization levels and cadmium stress were studied. The results showed that contents of chlorophyll a, chlorophyll b and carotenoids of S. nigrum under cadmium stress increased with the increasing concentration of nitrogen supplement. S. nigrum plants under cadmium stress and nitrogen supply had higher maximal photochemical efficiency of PSII in the dark, photochemical efficiency of PSII in the light, electron transport rates, total photosynthetic pigment molecules, total photosynthetic pigment molecules in the excited state k, photochemical reaction, but lower nonphotochemical quenching, eigen-absorption cross-section of photosynthetic pigment molecule, effective optical absorption cross-section of photosynthetic pigment molecules, thermal energy dissipation compared with plants under cadmium stress only. In conclusion, chlorophyll synthesis and PSII function of S. nigrum under cadmium stress were enhanced by nitrogen supply, and 300–450 mg/kg CO(NH ₂) ₂ is the suitable concentration.