Abstract: The aim of this study was to study the influence of the different seedling density, water and fertilizer(N,P,K) coupling effects on water use efficiency of fodder millet (Setaria itlica(L)Beauv) in autumn idle land. Five factors quadratic regression orthogonal rotation combination was designed in pot experiment with Jigu No.18. The results showed that moisture content and P2O5 had important effects on water use efficiency .The biggest influence on water use efficiency is moisture content , followed by P2O5.The significantly different coupling strength orders are: K and Seedling density> N and P > Moisture content and P > Moisture content and Seedling density > Moisture content and K. Moreover. Have built mathematical model:Y=44.26-1.311x1-2.298x2-3.682x3-6.401x4-34.540x5 0.273x1x3 0.118x1x4 0.843x1x5-1.948x2x3 6.631x4x5, the optimum proposal with economic benefit as evaluation indexes was determined as follows: Soil moisture conservation 10% and forage millet seedling density 15 kg/hm2. In the proposal, water use efficiency were 26.24 g/kg and hay grass yields were 13980.90 kg/hm2, economic benefit is 13830.90 yuan/hm2, which increased by 3063.73 yuan/hm2, the increased range was 22.15% when comparing with the optimal combination of the highest yield and increased by 6215.15 yuan/hm2, the increased range was 44.94% when comparing with the optimal combination of the highest water use efficiency. The results provided a theoretical basis and technical support to the fodder millet production in autumn idle land.