Abstract: In order to find out the appropriate ratio of ammonium/nitrate for plant growth and potassium absorption of flue-cured tobacco, the effects of different ratios of ammonium/nitrate on physiological growth of root, leaf photosynthesis, potassium content and absorption kinetics of tobacco plants at two potassium supply conditions were studied under hydroponic conditions with Yuyan No.6 as the material. The results showed that (1) Low potassium inhibited root physiological growth of tobacco seedlings. Root dry weight, root configuration indexes and physiological characteristics of tobacco seedlings increased first and then decreased with the decrease of ammonium/nitrate ratio at both potassium levels, and the highest content was found when the ammonium/nitrate ratio was 25/75. (2) The photosynthetic efficiency of tobacco plants under two potassium levels could be improved at an appropriate ratio of ammonium/nitrate. Under low potassium level, the chlorophyll content and photosynthetic efficiency of tobacco leaves increased first and then decreased with the decrease of ammonium/nitrate ratio. When the ammonium/nitrate ratio was 25/75, the net photosynthetic rate of tobacco leaves was significantly increased by 152.35% and 56.08% compared with that of pure ammonium and pure nitrate treatments. (3) With the decrease of ammonium and nitrate ratio, the potassium content and potassium absorption kinetics V_max values increased. The potassium content and V_max were the highest when the ammonium/nitrate ratio was 25/75 at the normal potassium supply level. Under low potassium level, the potassium content in tobacco plants continued to increase with the increase of nitrate/ammonium ratio. Therefore, reducing ammonium/nitrate ratio could change root architecture, promote root growth, and increase plant potassium content of tobacco plants. Meanwhile the inhibition effect of potassium deficiency on the growth and potassium absorption of tobacco plants could be alleviated. When the ammonium/nitrate ratio was 25/75, the comprehensive character of the tobacco plants was optimal.