In order to examine the role of environmental factors affecting foliar morphology, we performed a case study of leaf morphological variation of Ranunculus natans found in the arid zone of northwest China. We found that foliar phenotypic variation differed significantly between populations. We described substantial positive correlations between altitude and leaf area (LA) as well as leaf perimeter (LP), and also between longitude and number of teeth, along with dissection index (DI). The pH, conductivity, and salinity of the environment caused a significant decrease in both LA and LP. Ranked in terms of their impacts on leaf morphology, the six selected factors were: altitude > pH > conductivity > salinity > longitude > latitude. We found that foliar morphological variations are functional responses to water-quantity factors (e.g., altitude and longitude at regional scales) and water-availability relation factors (e.g., pH, conductivity, and salinity at local scales), rather than to temperature-relation factors (latitude). Therefore, altitude and longitude, along with pH, conductivity, and salinity, are the main factors that significantly influence foliar morphology in the arid zone of China. We found that main factors played major roles in plant phenotypic plasticity in a complex ecosystem, although different combinations and interactions of environmental and geographical factors in each local environment may obscure the general trends in trait changes along environmental gradients.