Differences in drought tolerance were studied among three strains of black locust (Robinia pseudoacacia L.) clones (one-year-old micropropagated K2, K3 clones and a common diploid clone CK) under simulated natural drought conditions. The experiment was carried out as a randomized complete blocks design. The following physiological variables were examined, biomass and allocation, photosynthetic characteristics, i.e. net photosynthetic rate (Pn), transpiration rate (E), stomatal conductance (Gs), intercellular CO2 concentration (Ci), instantaneous water use efficiency (WUEI), and long-term water use efficiency (WUEL). Results showed that the K2 and K3 had greater biomass, with the K3 displaying favorable ratio of root biomass to shoot and leaf biomass. Net photosynthetic rate (Pn) declined along with Gs, Ci and the ratio intercellular/ambient (Ci/Ca), under increasing drought stress. The decline in photosynthesis was mainly due to stomatal limitation. The K2 and K3 showed higher WUE and Pn than the CK. There were significant differences of drought level on WUEL, and the WUEL of K2 and K3 was greater than that of CK. Evaluation based overall physiological characteristics indicates the K2 and K3 are more drought tolerant, which are well adapted to the drought-stricken area of Northwest China.