Abstract: Abstract: A machine-transplanted large-pot carpet seedling has been one of the most innovative mechanical transplantings in precision agriculture. This study aims to explore the effects of seedling age and substrate soil ratio on the population quality and yield of late rice during transplanting. The optimal seedling age and substrate seedling raising technology were determined for the machine-transplanted late rice. The conventional indica-type rice (Sui Xiangnongzhan) was selected as the research object in this study. Three kinds of seedling age (25, 30 and 35 d), three kinds of composite substrate and natural soil ratio (substrate soil ratio 1∶1, 2∶1 and pure substrate) were used to clarify the effects of seedling age and substrate soil ratio on the seedling quality, machine-transplanted quality, population quality, yield, and the components of yield. The results showed that the substrate bulk density was significantly higher in the substrate soil ratio of 1:1 and 2:1 than that of the pure substrate, whereas, the water content, porosity, pH value, organic matter, and available nutrient elements were relatively lower than those of pure substrate treatment. A total porosity of 57.21% not suitable seedling growth was found in the substrate soil ratio 1:1 treatment. The available nitrogen contents were 223.62 and 331.25 mg/kg in the substrate soil ratio 1:1 and pure and substrate treatment, respectively, indicating the beyond seedling growth range. Moreover, the seedling quality performed better with the extension of seedling age, which was more conducive to the formation of strong seedlings. The seedling quality, population quality, yield, and the components of yield were relatively better in the substrate ratio 2:1 treatment, compared with the rest two substrates. The maximum rooting entwining force of 30 d seedling age was 73.19 N in the substrate ratio 2:1 treatment. A better performance was achieved in the pure substrate treatment, in terms of the planting percent, the basic seedlings number, and the pass rate of the relative uniformity of 35 d seedling age. Two treatments fully met the requirements of machine transplanting, according to the seedling missing, floating, injury, and overturning rate. The optimal seedling age of transplanting was obtained as 35 d in the substrate soil ratio of 2:1 treatment. There was a better performance in the tiller number, effective tiller number, earbearing tiller percentage, dry matter accumulation, and the harvest index of the population in each period after transplanting, compared with the short seedling age treatment. It infered that the long seedling age greatly contributed to the high yield population of machine-transplanted late rice. With the extension of seedling age, the seedling quality was better, which was more conducive to the formation of strong seedlings. An optimal grain maturity was achieved in the 35 d seedling age treatment, where the actual yield was significantly higher by 42.5% and 19.4%, respectively, compared with the 25 and 30 d treatment, and the moisture content of wet valley was significantly lower by 2.67 and 1.29 percentage points respectively. The grain maturity of 35 d seedling age treatment was better. The maximum correlation coefficient was 0.971 between the effective panicle number and the actual yield. The high yield of long seedling age treatment was attributed to the increase of effective panicle number. Therefore, the optimal seedling age and substrate seedling treatment were 35 d and substrate soil ratio 2:1 for the large-pot carpet seedlings of machine-transplanted late rice, when taking the twice chemical control. The findings can also provide a strong reference for the large-pot carpet seedling of machine-transplanted late rice in the southern double cropping rice areas, particularly for the higher level of planting mechanization.