The soybean yield formation in response to the elevated CO_2 (eCO_2) was analyzed in open top chamber (OTC) simulation systems using 8 soybean cultivars which were released in 1950-2006. Results showed that above ground biomass increased by 46.1% and yield increased by 37.8% across 8 soybean cultivars when the concentrations of atmospheric CO_2 increased up to 550 mumol ? mol~(-1). The yield response to eCO_2 varied in the 8 soybean cultivars. The minimum increase was only 0.6% in Hejiao 4 hao, while the maximum was 95.3% in Xiaohuangjin. The effect of eCO_2 on harvest index was not significant except for Suinong 9 hao. Yield components of soybean were also increased under eCO_2. Average plant height of 8 soybean cultivars was increased by 15 cm, while the soybean node number did not change significantly (P >0.05). The pod number and seed number increased by 42.9% and 37.0% under eCO_2 on average, respectively. This study also found that the contribution of yield components to soybean yield increase under eCO_2 differed. Under eCO_2, the increase of the 3-seed pod number,total pod number and seed number were significant correlated (P < 0.05). Due to the significant contribution of seed number to yield, the increase of yield under eCO_2 could be estimated by a model of y =0.175x +0.102 (P < 0.05; R~2 = 0.792),which was obtained through stepwise regression analysis.