Whether plants are able to adapt to environmental changes depends on their genetic characteristics and phenotypic plastic responses. We investigated the phenotypic responses of 7 populations of an important dominant species in semi-arid steppe of China - Stipa grandis, and then distinguished which adaptive mechanism(s), phenotypic plasticity or local adaptation, was/were involved in this species to adapt to environmental changes. (1) All traits were significantly influenced by the interaction of population and growth condition and by population in each condition, and inter-population variability (CVinter) was larger in the field than in the common garden for 8/9 traits, indicating that both phenotypic plasticity and genetic differentiation controlled the phenotypic differences of S. grandis. (2) From a functional standpoint, the significant relationships between the values of traits in the common garden and the environmental variables in their original habitats couldn't support local habitat adaptation of these traits. (3) Low CVintra, low quantitative differentiation among populations (Q ST ), and low plasticity shown in the western populations indicated the very low adaptive potential of S. grandis to environmental changes. (4) From the original habitats to the common garden which is far away from S. grandis distribution region, positive phenotypic responses were found in several populations, indicating that some original habitats have become unfavorable for S. grandis.