Abstract: In order to clarify the most suitable period of Melilotus officinalis (L.) Pall ploughing as a green manure in tobacco-growing areas and the appropriate amount of chemical fertilizers for flue-cured tobacco, field experiments were conducted to study the effects of Melilotus officinalis (L.) Pall at three stages (vegetative growth period, initial flowering period, and full flowering period) and combined with chemical fertilizers. The effects of three dosages (no fertilization, 70% constant fertilization and constant fertilization) on the growth and yield of flue-cured tobacco were analyzed. The results showed that: under the same fertilizer dosage, with the delay of the ploughing period, the net photosynthetic rate, transpiration rate and leaf carboxylation efficiency of flue-cured tobacco were significantly increased and the intercellular carbon dioxide concentration was significantly decreased. The full flowering stage was the best, with the proportion of upper medium tobacco, yield and output value at the full flowering stage significantly increased by 18.09%, 20.00% and 36.29% respectively compared with the vegetative growth period. During the same period of ploughing, with the increase of fertilizer dosage, the photosynthetic characteristics and economic traits of flue-cured tobacco increased first and then decreased, all reached the maximum value of constant fertilizer application at 70%. It can be seen that on the basis of ploughing Melilotus officinalis (L.) Pall, the photosynthetic intensity of flue-cured tobacco could be improved by delaying the ploughing period and reducing the amount of chemical fertilizer, promoting the accumulation of dry matter, with obvious value-adding effects. It was suggested that 70% constant (30% reduction) of chemical fertilizer application rate should be adopted at the full flowering stage.