<p>The development of ultrashort-duration early-season rice varieties is critical for advancing the oilseed rape–double-season rice system, which is promoted by the Chinese government to increase domestic oil crop supply without reducing rice production. This study aimed to evaluate yield performance and identify key agronomic traits contributing to high grain yield in late-sown ultrashort-duration early-season rice adapted to this system. Field experiments were conducted in 2024 and 2025 to assess growth duration, grain yield, daily grain yield, biomass production and translocation characteristics, and their relationships in five ultrashort-duration early-season rice varieties. All five varieties completed their growth cycle within 100 d in both years, with Nanlingzao 2 (NLZ2) exhibiting the shortest duration—87 d in 2024 and 90 d in 2025. Significant varietal differences were observed in 2024, with Yuezao 6, Zhenzao 1930, and NLZ2 yielding 36%–45% more than Jinchaozao; however, no significant differences among varieties were detected in 2025. Relationship analysis showed that higher daily grain yield, greater total biomass production from improved pre- and post-heading growth, and a higher harvest index driven by enhanced post-heading growth were the main contributors to higher yield in ultrashort-duration early-season rice. These findings provide important insights for breeding and crop management strategies to improve the performance of ultrashort-duration early-season rice within the oilseed rape–double-season rice system.</p>