The effect of nitrogen application on photosynthetic characteristics of alfalfa (Gannong No.3 and Gannong No.9) and their resistance to Odontothrips loti were studied through 4 nitrogen application treatments (N1,N2,N3,N4) and one control (N0).The damage degree of alfalfa caused by O.loti at different nitrogen application levels was evaluated,at the same time,the chlorophyll content and photosynthetic parameters of alfalfa were measured.The results showed that the damage index of alfalfa increased along with the increase of nitrogen application level.The chlorophyll content in leaves increased after nitrogen applicated,but it decreased along with the damage index.The net photosynthetic rate,transpiration rate,stomatal conductance and water use efficiency of alfalfa increased,and the intercellular CO2 concentration decreased after alfalfa was infested by O.loti for 7 days.The net photosynthetic rate,transpiration rate,stomatal conductance and intercellular CO2 concentration decreased,and water use efficiency increased after alfalfa was infested by O.loti for 14 days.It could be concluded that nitrogen application reduced the resistance of alfalfa to thrips.With the increase of the damage degree of alfalfa,the materials for photosynthesis were insufficient and this resulted in the decrease of resistance to thrips.