Abstract: Mechanical transplanting has been limited to the blanket seedling cultivation, due to the heavy weight of seedling substrates, high labor intensity, and the large number of seedling trays. In this study, the high-density seeding was employed to replace the conventional seedling soil with the lightweight crop straw boards. The mechanic transplanting was then enhanced the seedling quality and transplanting performance. The labor inputs were also reduced to promote the rice blanket seedling. The japonica rice cultivar ‘Nanjing 5718’ was taken as the test material. A conventional seeding density (D1: 150 g/tray) was evaluated under four high-density treatments (D2: 200 g/tray; D3: 250 g/tray; D4: 300 g/tray; and D5: 350 g/tray) and four seedling ages (A1: 10 d; A2: 15 d; A3: 20 d; and A4: 25 d). The investigation focused mainly on the seedling growth, physiological attributes, mechanical transplanting quality, yield, and economic returns. Results indicated that the key indicators of seedling quality—such as the soluble sugar content, root activity, and rooting ability—declined with increasing seedling age under high-density treatments (D2–D5). These indices were even better at the shorter seedling ages (10 and 15 days) than those of the conventional combination (D1A3). The high-density seeding was integrated with the vigor in the early growth stage, when the seedlings relied primarily on the nutrient reserves in the endosperm, together with the lower demands for the light and temperature. The higher activities of protective enzymes were observed under high density at 15 days, including the superoxide dismutase (SOD) and peroxidase (POD), indicating the enhanced stress tolerance; Whereas, the malondialdehyde (MDA) content was signified to heighten the oxidative stress, with the longer seedling ages, due to the intensified competition for the limited nutrients. Furthermore, the missing hill rate in the high-density treatments decreased by 19.78%–60.26%, compared with the conventional D1 treatment, thus leading to the more uniform pattern of mechanical transplanting. Although the seedling injury rate increased by 32.25%-177.0%, due to the increased inter-plant competition. The floating seedling and lodging rates were reduced by 15.01%-50.55% and 17.53%-45.44%, respectively, during mechanical handling. The transplanting quality was shorten the recovery period post-transplant and support sustained crop growth. The ultrahigh density was also reduced the number of seedling trays per unit area, which was dropped by 12.12%–58.98%, compared with the conventional. The lower labor and material costs were directly reduced correspondingly. Notably, the D4A2 combination (300 g/tray with a 15-day seedling age) yielded the best overall performance, thus producing 10.69 and 10.65 t/hm² over two consecutive years, with the highest economic returns of 32 085.32 and 31 967.25 Yuan/hm². In summary, the high physiological quality and mechanical performance of transplanting were achieved in the ultrahigh density seedling cultivation with the crop straw boards under shorter seedling age. The required number of seedling trays was effectively reduced to enhance the overall economic benefits. The synergistic effect between high-density seeding and early transplantation can provide a promising strategy to advance the mechanized rice production in the more sustainable, cost-effective agricultural practices. In conclusion, the lightweight crop straw boards with the ultrahigh density seeding at the optimal seedling ages were significantly enhanced the efficiency and sustainability of mechanized rice cultivation