Water and phosphorus (P) are two major factors affecting plant growth on the Loess Plateau of China. To clarify root response of introduced species to native species in mixture under varying water and P supplies would be favorable for assessing their interactions. This study investigated the effects of soil water, P and mixture ratio on root growth and morphology of switchgrass (Panicum virgatum L.) (an introduced C4 perennial herbaceous grass) and bushclover (Lespedeza davurica S.) (a C3 perennial leguminous subshrub) in mixtures in a pot experiment. Two soil water regimes [75 ± 5% field capacity, FC (high water, HW) and 35 ± 5% FC (low water, LW)], three P treatments (addition of 0, 0.05 and 0.1 g of P2O5 per kg dry soil) and 5 mixture ratios of the two species (12:0, 8:4, 6:6, 4:8, 0:12) were complemented. Results showed that switchgrass tended to decrease root average diameter (RAD) and increase specific root length (SRL) under LW comparing with HW, while bushclover showed the opposite trends. P application significantly (p<0.05) deceased the RAD of switchgrass under LW, while had no effect on SRL, resulting in a thinner root system with higher root tissue density. Regardless of soil water regimes, SRL of bushclover decreased significantly after P application. Root biomass and total root length of switchgrass were increased with the decrease of mixture ratio, while both parameters decreased in bushclover. In mixture with switchgrass, bushclover decreased SRL and proportion of fine root length (0-0.5 mm diameter class). The present study implied that switchgrass would be superior when mixed with bushclover, and P application might increase the abilities of switchgrass in competing and acquiring for limited resources.