Abstract: 【Objectives】Both the nitrogen application rate and transplanting density affect the formation of good population structure and rice yield. The suitable coupling of the two factors are the base of reaching highest yield and nitrogen use efficiency. An experiment was carried out in low-yield paddy field to setup the optimal nitrogen application rate and transplanting density.【Methods】Regarding low-yield paddy as the object of this research, an interactive test of four levels of nitrogen application rate (0, 135, 180, 225 kg/ha) and three gradients of transplanting density (12104, 16.5104, 21104 holes/ha) were conducted with a rice cultivar of Liangyoupei9 as tested material. The items are investigated including stem and tillerring dynamics, leaf area indices in different growth periods, the nitrogen contents in grain and straw, and the yield and yield components. The optimum structure index when the yield reaches the highest was calculated by the curvilinear regression of yield along with LAI, TAB, SPP and PN. The nitrogen use efficiencies were compared among the treatments.【Results】Both the N application rate and transplanting density have significant effects on yield, and effect of interaction exists between them. 1) Yield and total aboveground biomass are improved with the increase of nitrogen application rates and transplanting densities. When the nitrogen application rate reaches 225 kg/ha, harvest indices decrease, instead. The harvest index decrease with the increase of transplanting density. 2) The rice production is closely related to panicles per square meter, sword leaf area in filling stage and shoot dry weight. Quadratic equation best describes the correlations with the highest correlation coefficients. Calculated with the equation, the theoretical highest yield will be obtained when the sword foliage area index in filling stage is 1.36, the above ground dry weight is 24371 kg/ha, the panicle numbers per square meter is 338, the spikelet number per panicle is 195, respectively. 3) N agronomic efficiency (NAE), N partial factor productivity (PFPN), N recovery efficiency (NRE) and N physiological efficiency (NPE) are significantly decreased with the increase of nitrogen application rates, particular for NAE, PFPN. Increasing transplanting density could obviously improve the NAE, PFPN and NRE, but had not on NPE.【Conclusions】With less nitrogen application rate, higher yield can be acquired by increasing transplanting density. In the tested soil condition and the nitrogen application rate of 180 kg/ha, the suitable transplanting density can be as high as 21104 holes/ha for the highest grain yield.Under the combination, the population structure indices and the nitrogen efficiency indices are all the highest. So the N application rate of 180 kg/ha with transplanting density of 21104 holes/ha are the optimal choice in the low-yield paddy field in the tested area.