Abstract: STK12 encodes serine/threonine protein kinase in soybean, which plays an crucial role in soybean may regulate a lot of growth and development process and abiotic stress. In this study, the full length of soybean GmSTK12 was isolated, and GmSTK12 was constructed in the plant expression vectors pTF101 by homologous cloning method was successfully constructed. The soybean variety "Dongnong 50" was genetically transformed by Agrobacterium-mediated cotyledonary node transformation method, and three genetically modified（GM） soybean events were obtained. Through the investigations of T₁ and T₂ generation materials of genetically overexpressed soybeans, the results showed that overexpression of GmSTK12 gene increased the soybean leaf length-width ratio, number of lateral roots, and root length significantly compared with the control; the leaf area increased from the V6 stage and was about twice that of the control; the overexpression of GmSTK12 gene made the fresh and dry weights of aboveground parts and roots were significantly higher than those of the control as well as the chlorophyll content was significantly increased. Taken together, the results suggested that GmSTK12 redundantly and differentially enhanced the soybean biomass.