Using conventional late japonica rice Xiushui 134, Xiushui 14 and Jia 67 as materials, which were widely used in Tongxiang City, the effects of different leaf age and plant spacing on the yield and composition and the dynamics of stems and tillers of conventional late japonica rice were studied. The results showed that, the number of effective panicles and total spikelets decreased significantly, the number of grains per panicle and the thousand-grain weight increased significantly, and there was no significant difference in seed setting rate among different treatments with the extension of the transplanted seedling age. The yield of machine-transplanted conventional japonica rice at 3.0 leaves treatment was the highest（9.73 t/hm²）, which was 5.99% and 15.14% higher than 3.5 leaves and 4.0 leaves treatments, respectively. With the decrease of planting density, the number of effective spikes, total spikelets and 1000-grain weight all decreased significantly, while the number of grains per spike did not increase significantly. The average yield of machine-transplanted conventional japonica rice（9.34 t/hm²） under the 14 cm plant spacing treatment was the highest, which was 2.86% and 10.66% higher than that of the 16 cm and 18 cm treatments, respectively. In conclusion, the seedling age of 3 leaves with 30 cm×14 cm spacing seems to be the optimal seedling age and planting density for machine-transplanted conventional japonica rice.