Abstract: Abstract: The high proportion of early advantaged tillering in bowl seedling mechanical-transplanting rice promotes individual growth and improves canopy's efficiency of solar energy utilization, which is beneficial for large panicle formation and leads to significant increase in the total spikelet number. However, the related physiological mechanism remains unclear. In order to explore the physiological reasons for the formation of large panicle of bowl seedling mechanical-transplanting rice, with conventional blanket seedling mechanical-transplanting as the control (CK), the hybrid japonica rice variety with large panicle Yongyou 1540 was planted in the field to investigate the features of carbon (C) and nitrogen (N) metabolism in vegetative organs, and the differentiation and degradation of branches and spikelets in young panicle differentiation stage of bowl seedling mechanical-transplanting rice. Their regularity and differences were studied, and their relationships with yield components were investigated. The results showed that the contents of non-structural carbohydrates (NSC) in vegetative organs under the 2 mechanical-transplanting methods were obviously different. The content of NSC in bowl seedling mechanical-transplanting was significantly higher than that in blanket seedling mechanical-transplanting in every growing period in panicle differentiation stage. On the 25th and 20th day before the heading, the total N content in bowl seedling mechanical-transplanting was higher than that in blanket seedling mechanical-transplanting, but the trend became opposite since the 15th day before the heading when the C/N ratio also became significantly higher than that of blanket seedling mechanical-transplanting. From the 25th to the 15th day before the heading, the difference of NSC accumulation under the 2 methods was not obvious, but from the 10th day before the heading to the heading, NSC accumulation in vegetative organs increased rapidly and that of bowl seedling mechanical-transplanting was significantly higher than that of blanket seedling mechanical-transplanting. The number of spikelets was closely related to the formation of branches, and the influences of the two mechanical-transplanting methods on the formation of branches and spikelets were consistent, which showed that the number of differentiated and survived branches and spikelets in bowl seedling mechanical-transplanting was significantly higher than that in blanket seedling mechanical-transplanting, while the number of degradation and the degradation rate had an opposite trend. Especially, the influences on the number of secondary spikelets reached a significant or extremely significant level, and the influence degree on the formation of secondary branches and spikelets in the basal part was higher than that in the middle and upper part. The C/N ratio in vegetative organs on the 25th and 20th day before the heading was in a significant or extremely significant negative correlation with the number of differentiated/survived branches and spikelets, especially the secondary branches and spikelets, and was in a positive correlation with the number of degradation, which was contrary to the situation from the 15th day before the heading to the heading. The 1000-seed weight and yield were both in a significant or extremely significant positive correlation with the NSC accumulation in young panicle differentiation stage and the NSC translocation rate after the heading. In bowl seedling mechanical-transplanting, vegetative organs could achieve a high level of C/N ratio and NSC accumulation, which promoted the differentiation of branches and spikelets, reduced the degradation, increased the grain number per spike and finally led to a higher grain yield than the blanket seedling mechanical-transplanting. This study provides a reference for the matching technology of rice mechanized production and a theoretical foundation for high yield and efficient production of hybrid rice.