Global climate change poses a severe threat to mountain biodiversity. Phenotypic plasticity and local adaptation are two common strategies for alpine plant to cope with such change. They may facilitate organismal adaptation to contrasting environments, depending on the influences of the environment or genotype or their interacted effects. In this study, we use an endemic alpine plant (Rorippa elata) in the Hengduan mountains (HDM) to unravel its phenotypic basis of adaptation strategy and evaluate the relative contributions of environment and genotype to its phenotype. We transplanted 37 genotypes of R. elata into two common gardens across low and high elevations (2800 vs. 3800 m) during 2021–2022. Nine fitness-related traits were measured, including flowering probability and glucosinolates (GS) content. We estimated the environmental or genotypic contributions to the phenotype and identified the main environmental components. Our results revealed that both environment and genotype-by-environment interactions contributed to the phenotypes of R. elata. Latitudinal heterogeneity was identified as a key factor that explained 24% of the total phenotypic variation. In particular, genotypes of the northern HDM showed significantly higher plasticity in flowering probability than those of the southern HDM. Furthermore, within the southern HDM, GS content indicated local adaptation to herbivory stresses for R. elata genotypes along elevations. In conclusion, our results suggest that R. elata may have adapted to the alpine environment through species-level plasticity or regional-level local adaptation. These processes were shaped by either complex topography or interactions between genotype and mountain environments. Our study provides empirical evidence on the adaptation of alpine plants.