Seed length and seed width are an important factor to the soybean yield. So the quantitative trait loci (QTL) location for seed length and seed width could assistant the breeding of soybean. In this study, the QTL underlying seed length and seed width were studied. A recombinant inbred line population of soybeans derived from a cross between the American semi-draft cultivars Charleston and Dongnong 594 were used in 7 environments. The quantitative trait loci underlying seed length, seed width, and seed length/seed width were analyzed by the method of composite interval mapping. Then, the epistatic effects and the QTL-environment (QE) interaction effects were also analyzed. Some valuable QTL sites found had great effect to the seed trait. Results showed that 7 QTLs underlying seed length were identified mainly on linkage groups D1a, C2, B1, A1, G, and A2. For the seed width, 7 QTLs were identified on linkage groups D1a and O. Two QTLs of seed length/seed width were identified on linkage groups D1b and C2. No QE interaction was found for QTLs of seed length and seed width in 7 environments. QTLs of seed length/seed width on linkage groups A1 and I had a QE interaction in 7 environments. Seven pairs of QTLs were identified that affected additive x additive epistatic effect of seed length, seed width, and seed length/seed width, which occurred among 8 linkage groups. These results supply a good foundation for molecular assistant breeding for soybean seed trait.