This study was conducted to evaluate the effects of biochar (BC) application on the active organic carbon and microbial activity in the soil, and crop growth, as well as the action mechanism between biochar and water-stable aggregates (WSA) in the soil. Biochar and straw were added at 1%, 3%, and 10% in straw treatments (Str_1.0, Str_3.0, and Str_10.0, respectively) and 0.8%, 2.4%, and 8% in biochar treatments (BC_0.8, BC_2.4, and BC_8.0, respectively). With biochar and straw applied at equal C rates, the wet sieving method, scanning electron microscope, and kinetic model equation were used to investigate the responses of enzymatic activity, soil structure, active organic carbon, and wheat yield. The results indicated that the proportion of macro-aggregates (> 0.25 mm) was significantly increased by 16.9%-45.8% in biochar treatments, while the proportion of macro-aggregates was double that of micro-aggregates in the treatment where straw was applied in excess. Soil microbial biomass C content was increased by 9.7%-33.6%. However, the dissolved organic C content was decreased by 12.6%-27.5% in BC_0.8 and BC_2.4, while the reverse was observed in BC_8.0. The kinetic parameters of the enzyme (michaelis constant, maximum enzyme velocity, and velocity constant) were decreased by 17.3%, 17.0%, and 16.1% in biochar treatments, respectively. Compared with the control, wheat yields were increased by 14.9%-19.1% in BC_0.8 and BC_2.4, while it decreased by 37.3% and 90.1% in Str₃ and Str₁₀, respectively. In general, soil structure and wheat growth were promoted by increasing the formation of soil WSA fractions that were >0.25 mm and invertase activity. Biochar applied at the rate of 2.4% was beneficial for improving the soil structure and crop growth, and is effective for the utilization of excess plant residues in the study area.