Extending the seedling age can lead to a decrease in the yield of mechanically transplanted early rice, while different planting densities significantly affect the production of machine-transplanted rice with extended seedling age. This study aims to clarify the effects of different planting densities in alleviating yield loss caused by prolonged seedling age. This experiment used the conventional early indica rice variety Zhongzu 143 as the research subject. Two different seedling ages for mechanical transplantation were established: 20 days (B1) and 35 days (B2), and three planting specifications for mechanical transplantation were set: 30 cm × 12 cm (D1), 30 cm × 16 cm (D2), and 30 cm × 18 cm (D3). The changes in tillering dynamics, dry matter accumulation, and yield as well as its components of rice populations under different planting densities were investigated. The results showed that for early rice with extended seedling age under mechanical transplantation, the dry matter accumulation decreased, tillering slowed down, and the number of effective panicles reduced, leading to a decrease in yield. For early rice with extended seedling age under mechanical transplantation, increasing the planting density could enhance the number of effective panicles and dry matter accumulation, thus increasing the yield. In summary, increasing the planting density can compensate for the reduction in the number of effective panicles and dry matter accumulation under extended seedling age, thereby effectively alleviating the yield loss associated with mechanical transplantation of rice with extended seedling age.