The photosynthetic characteristics of Bothriochloa ischaemum and Lespedeza davurica in the loess hillgully was investigated under potted condition, the photosynthetic gas exchange parameters of two species during the heading period, flowering period and mature period were studied under three soil water regimes (HW, 80％±5% of field capacity(FC); MW, 60％±5% of FC; LW, 40％±5% of FC) and seven plant mixture ratios (i.e. 0∶12, 2∶10, 4∶8, 6∶6, 8∶4, 10∶2 and 12∶0, respectively). The objectives were to determine the leaf photosynthetic characteristics of two species across their growth periods under different mixture ratios, and to identify their coexistence mechanisms of two native species under lower water availability. The results showed that: (1) the net photosynthetic rate (P_n), stomatal conductance (G_s) and transpiration rate (T_r) of two species were decreased under water stress, and the photosynthetic response of B. ischaemum was more sensitive to water stress; water stress decreased photosynthesis of both species during growth periods were caused by stomatal limitation, except for the species of L. davurica in the mature period under LW soil water regime; (2) under HW soil water regime, the averaged P_n values of B. ischaemum in mixtures were significantly improved, whereas adversely affected L. davurica throughout the growing season; Under MW or LW soil water regimes, P_n in mixtures showed higher values when B. ischaemum in heading period or flowering period and L. davurica in mature period, compared to respective monoculture, and the water use efficiency (WUE) of B. ischaemum were significantly higher than that of L. davurica in different growth periods; (3) when the B. ischaemum and L. davurica at a mixture ratio of 10∶2, the P_n and WUE of B. ischaemum in different growth periods under LW regime, and the P_n and WUE of L. davurica in mature period under the MW or LW regimes, which were significantly higher than that of the species in monoculture. These results suggested that the B. ischaemum and L. davurica at a mixture ratio of 10∶2 improved the leaf photosynthesis and water use efficiency of both species under low soil moisture condition.