[Objective] The effects of exogenous application of corn straw biochar on the soil enzyme activity and microbial community structure were studied to provide soil conditioner for ecological restoration of crude-oil-contaminated soil in Longdong area of the Loess Plateau.[Methods] Three treatments, namely corn straw biochar treatment (B), Calendula officinalis treatment (J), and Calendula officinalis plus corn straw biochar treatment (JB) were applied to the low-concentration (5%) and high-concentration (20%) crude-oil-polluted soil. The change of the soil total petroleum hydrocarbon (TPH) removal rate, the soil enzyme activity, and the soil microbial community structure was investigated by the conventional method.[Results] ① The maximum soil TPH removal rates for the low-pollution-concentration group (TPH=5%) were observed in JB₅ treatment (61.95%±1.39%), whereas the JB₂₀ treatment (56.44%±1.89%) demonstrated the maximum soil TPH removal rate, in the high-pollution-concentration (TPH=20%) group (p<0.05). ② Soil dehydrogenase and polyphenol oxidase activity significantly improved after the application of corn straw biochar. ③ When the crude-oil concentration increased from 5% to 20%, the difference in soil microbial community structure between JB₂₀ and B₂₀ treatments was relatively small. ④ Nonmetric multidimensional scaling analysis revealed that the difference in soil microbial community structure between the treatments of J₅, B₅, B₂₀, JB₅, and JB₂₀ and the treatments of CK₅, CK₂₀, and J₂₀ was caused by the changes of soil dehydrogenase activity, soil polyphenol oxidase activity, Chao1 index, and Shannon-Wiener index of the soil microbial community.[Conclusion] Application of corn straw biochar to crude-oil-polluted soil with low and high concentrations significantly improved the soil dehydrogenase and polyphenol oxidase activity, causing changes in the soil microbial community structure. Consequently, the soil TPH removal rate improved. Therefore, corn straw biochar can be used as a soil conditioner for ecological restoration of crude-oil-contaminated soil in Longdong area of the Loess Plateau.