Purpose <p>The global rise in human population and the decline of arable land pose a significant threat to agricultural sustainability. Rapeseed (<i>Brassica napus</i> L.), the world’s third-most important oilseed crop, has a characteristically low harvest index (HI), limiting its yield potential. We hypothesize that selecting genotypes with a higher HI under dense, direct-seeding conditions will enhance dry matter and ¹³C assimilation and translocation, ultimately resulting in a substantial increase in seed yield.</p> Methods <p>A two-factor split plot design experiment with two different types of harvest index genotypes (Type 1; relatively low harvest index genotypes (Za 1945 and Zhongyouza 19) and Type 2; relatively high harvest index genotypes (Jiayou1hao and Nanyou 6211), two planting densities (traditional direct seeding planting density (LPD) = 150000 plantsha^− 1 and High planting density (HPD) = 300000 plantsha^− 1) was conducted in 2020–2022 years.</p> Results <p>We found that type 2 genotypes performed better at high planting density as compared to type 1 genotypes. We observed that the transportation of ¹³C and dry matter was restricted in low harvest index genotypes under both planting density, and more carbon and dry matter were transported from stem and leaves to grain of type 2 genotypes. Additionally, a higher dry matter contribution to grain was observed in high harvest index genotypes under high planting density.</p> Conclusion <p>This study demonstrates that selecting high-harvest-index genotypes is a viable strategy to maximize rapeseed yield under dense planting. This approach provides a practical pathway for agricultural intensification, allowing for more efficient use of limited land resources and making a valuable contribution to global food security.</p>