Abstract: The objective of this experiment was to investigate the effects of different nitrogen forms and ratios on photosynthetic characteristics of Brassica pekinensis， and the correlation between photosynthetic characteristics and nitrate ammonium ratios were analyzed. The results showed that chlorophyll a， chlorophyll b， chlorophyll a+b and carotenoid content of Brassica pekinensis were higher than the control （CK1） in different treatments and growth stages， and there were large significance among different treatments especially in the rosette stage and cabbage stage. The chlorophyll contents in the treatment with nitrate ammonium ratios of 7∶3 and 5∶5 were higher than others， which were beneficial for the accumulation of chlorophyll. Moreover， the treatment with nitrate ammonium ratios of 7∶ 3 and 5∶5 and CK2（ local fertilization treatment） were conducive to the absorption， transmission and conversion of light energy. It could be concluded by quadratic polynomial curve regression that there were dependency relationships between ammonium nitrate ratios and the actual photochemical efficiency （ФPSⅡ）or net photosynthetic rate （Pn）. When the ammonium nitrate ratios were 7∶3 and 5∶5， it were more beneficial for light energy conversion and CO2 fixation in Brassica pekinensis.

Key words: Brassica pekinensis, nitrogen, chlorophyll, photosynthetic characteristics