The field trail was conducted by setting five different planting densities 37.5×10⁴(Treatment A,TA),45.0×10⁴(Treatment B,TB),52.5×10⁴(Treatment C,TC),60.0×10⁴(Treatment D,TD) and 67.5×10⁴(Treatment E,TE) plants·hm^-2,under drip irrigation by using of randomized block experimental design in 2012 and 2013 summer soybean(‘Heihe 43’) growing seasons,and the leaf SPAD,leaf area index(LAI),specific leaf area(SLA),photosynthetic characteristics and yield components of summer soybean were observed.The result showed that:(1)The leaf SPAD value of summer soybean was less affected by density in seedling stage,but had significant differences among different treatments after seedling stage and increased with higher density in the whole growth period of summer soybean.(2)The LAI was also improved along with higher density in the two trials,and reached the highest in the seed filling period,and the maximum value of 2012 and 2013 in TE were 6.24 and 5.56,which was Significantly higher 18.41% and 36.17% than that of the lowest treatment TA at the same period.SLA increased with higher density,too,and had significant differences in the early and late growth stages while the difference was relatively small in middle growth stage.(3)The net photosynthetic rate(P_n),transpiration rate(T_r) and stomatal conductance(G_s) were increased firstly then decreased with density increase.P_n and G_s reached maximum in the pod-filling period,but T_r reached maximum in the flowering period;intercellular CO₂ concentration(C_i) was first decreased and then increased with density increase,TC was the lowest both in 2012 and 2013,185.70 μmol·mol^-1 and 179.61 μmol·mol^-1,respectively.(4)Treatment C obtained the highest yield of 3 205.04 kg·hm^-2(2012) and 3 142.53 kg·hm^-2(2013),which was higher 1.42%-14.26% and 1.08%-27.65% in 2012 and 2013,respectively.Therefore,the summer soybean with optimum density was beneficial to light utilization for the high chlorophyll content,and the optimum LAI without plant canopy closure of suitable density ensured the gas exchange between internal and external,which improved the photosynthetic rate and yield.