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Biological yield indicates the potential for increasing yield. Leaf carbon metabolism plays an important role in the biomass accumulation of rapeseed (Brassica napusL.). Field experiments with the hybrid HZ62 (with a conventional plant architecture) grown in 2016–2017, and HZ62 and accession 1301 (with a compact plant architecture) grown in 2017–2018 were conducted to characterize the physiological and proteomic responses of leaf photosynthetic carbon metabolism to density and row spacing configurations. The densities were set at 15 × 104ha−1(D1), 30 × 104ha−1(D2), and 45 × 104ha−1(D3) (main plot), with row spacings of 15 cm (R15), 25 cm (R25), and 35 cm (R35) (subplot). Individual and plant population biomass accumulation was greatest at R25, R15, and R15 for D1, D2, and D3, respectively, for both genotypes. In comparison with D1R25, the individual aboveground biomass of HZ62 decreased by 60.2%, whereas the population biomass increased by 31.9%, and the individual biomass of genotype 1301 decreased by 54.0% and the population biomass increased by 53.9% at D3R15. Leaf carbon metabolic enzymes varied between genotypes at flowering stage. In contrast to D1R25, at D3R15 the activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and sucrose phosphate synthase (SPS) and the contents of starch, sucrose and soluble sugars in leaves were significantly decreased in HZ62 and increased in genotype 1301. The activities of fructose-1,6-bisphosphatase (FBPase) decreased, in consistency with the abundance of fructose-bisphosphate aldolase in HZ62. In contrast, sucrose synthase (SuSy) activity appeared to decrease in both genotypes, but a significant increase in abundance of a protein with sucrose synthase was found in the 1301 genotype by proteomic analysis. With increased density and reduced row spacing, the expression of most key proteins involved in carbon metabolism was elevated, and enzyme activity and carbon assimilate content were increased in 1301, whereas HZ62 showed the opposite trend, indicating that the compact plant type can accumulate more population biomass with denser planting.