Strontium (Sr) is a common fission product of U-235 and Pu-239. Radioactive isotopes show heavy metal toxicity and radioactivity. The fate of radionuclides in the environment follows the behavior of stable elements; therefore, the behavior of stable Sr-88 should be regarded as a useful analog for predicting the fate of Sr-90 in the environment. Phytoremediation is an emerging technology whereby plants remove metals from the environment. The ultimate goal of phytoremediation is to eliminate or reduce soil pollutants and restore soil quality. The phytoremediation of contaminated soil can be assessed by evaluating the soil nutrient content and enzyme activity. Biolog technology is used to characterize the functional diversity of the soil microbial community and kinetic characteristics of carbon source utilization, and has been widely applied to determine the effect of heavy metal pollution on microbial communities. In our previous study, 26 cultivars of wheat (Triticum aestivum L.), oat (Avena sativa L.), and barley (Hordeum vulgare L.) were compared to investigate Sr accumulation and distribution for their potential use in phytoremediation. Based on these results, Neimengkeyimai-1 could be used as a model for further research, as a starting point for finding additional effective cultivars. In this experiment, A. sativa ‘Neimengkeyi-1’ was selected as the model plant to evaluate the effect of oat planting on Sr-contaminated soils. Through pot experiments, Sr was added to soils in six treatments:25 mg/kg Sr (CK), 100 mg/kg Sr (L), 250 mg/kg Sr (H), 25 mg/kg Sr + oat (CK+oat), 100 mg/kg Sr + oat (L+oat), and 250 mg/kg Sr + oat (H+oat), respectively. After 30 days of cultivation, the soil enzymes (including invertase, phosphatase, protease, urease, and dehydrogenase) and soil microbial community functional diversity were measured to evaluate the phytoremediation effect of oat on Sr-contaminated soils. The results showed that oat significantly improved the activities of urease and dehydrogenase. Oat significantly increased utilization of the average of 31 carbon sources (average well color development, AWCD) by the soil microbial community and improved the Shannon diversity index (H), Shannon evenness index (E), and carbon source utilization richness index (S), but reduced the Shannon dominance index (D) under Sr pollution. Principal component analysis (PCA) showed that the utilization of carboxylic and polymer carbon sources were, respectively, higher by 24% and 23% in L+oat treatments and by 24% and 13% in H+oat treatments than in non-planting treatments (L+H). The carboxylic and polymer carbon sources were the main carbon sources in the soil microbial community under Sr pollution. A cluster analysis showed that Sr pollution in soils was close to the metabolic soil microbial community characteristics, and planting oat significantly improved the soil environment. These results were consistent with those of soil enzyme activity and the PCA. In conclusion, oat increased enzyme activity and improved the functional diversity of rhizosphere soil microbes in Sr-polluted soil.