Abstract: In order to offer scientific base for cultivation and management of forest,effects of light radiation intensity on photosynthetic characteristics and water use efficiency of Platycladus orientalis and Pinus tabulaeformis were studied under different soil moisture content.Controlling soil moisture,under the simulated light photosyntyhetic radiation（SPR）,net photosynthetic rate （P_n）,transpiration rate（T_r）,water use efficiency（WUE） and intercellular CO₂ concentration（C_i） of Platycladus orientalis and Pinus tabulaeformis was studied in a semi-arid region of loess plateaus.The results showed that within the range of 0（2200）μmol photons/（m²·s）,P_n,Tr and WUE enhanced with an increase of the SPR for Platycladus orientalis and Pinus tabulaeformis,but P_n and WUE of Platycladus orientalis and Pinus tabulaeformis declined with continued increase of the SPR;P_n,T_r,WUE and light saturation point（LSP） of Platycladus orientalis was higher than Pinus tabulaeformis,while light compensation point（LCP） of Platycladus orientalis was lower than Pinus tabulaeformis in the same soil moisture,efficiency of light energy utilization of Platycladus orientalis was higher than Pinus tabulaeformis;P_n,T_r and Ci of Platycladus orientalis and Pinus tabulaeformis enhanced with an increase of the soil moisture,while WUE declined.The LSP of Platycladus orientalis was respectively（1275）,（1450） and（1675）μmol photons/（m²·s）,and LCP was respectively（4225） and 13μmol/（m²·s）,with the maximum P_n of 3.04,4.06 and 5.53μmol/（m²·s）,under the soil moisture of 7.90%,13.00% and 19.99%;The LSP of Pinus tabulaeformis was respectively（1100）,（1325） and（1500）μmol/（m²·s）,and LCP was respectively 60,30 and 23μmol/（m²·s）,with the maximum P_n of 1.08,3.35 and 4.36μmol/（m²·s）,under the soil moisture of 7.83%,13.04% and 20.15%.