[Objective] The effects of applying biochar combined with subsoiling on soil bulk density, soil structure stability, and organic carbon in farmland topsoil were comprehensively analyzed to provide scientific and reasonable control measures and a theoretical basis for soil structure improvement. [Methods] A field experiment was conducted from 2017 to 2019 based on a winter wheat and summer corn rotation system. Biochar was applied at rates of 0, 3 000, and 6 000 kg/hm² (B₀, B₁, and B₂) during the wheat season in conjunction with two tillage treatments 〔traditional stubble sowing (P) and subsoiling tillage sowing (S)〕 imposed during the corn season to produce a total of six combined treatments of B₀P, B₁P, B₂P, B₀S, B₁S, and B₂S. [Results] ① After two crop rotations, under the same tillage method, soil bulk density decreased with increasing biochar application rate, and bulk density under B₂S was significantly lower than under B₀S by 13.59%. Under the same biochar application level, soil bulk density under subsoiling was less than under traditional stubble sowing, and the subsoiling bulk density was 4.95% lower than the bulk density under traditional stubble sowing; ② After two years of continuous application of biochar combined with subsoiling tillage, the organic carbon content of B₂S was the highest, and was 42.15% greater than observed for B₀P and 1.03% greater than observed for B₂P; ③ After two crop rotations, the mechanical aggregate content (DR_0.25), the water-stable aggregate content (WR_0.25), and the mean weight diameter (MWD) showed an upward trend with increasing biochar application rate. In contrast, soil aggregate destruction rate (PAD) and soil unstable aggregate index (ELT) showed a decreasing trend, and B₂S produced better results than the other treatments, indicating that high biochar application rates combined with subsoiling tillage more effectively improved aggregate stability than traditional tillage. [Conclusion] A basic application of 6 000 kg/hm² of biochar during the wheat season combined with corn season subsoiling could improve the stability of soil aggregates and the soil structure of farmland in the North China Plain.