In this study, we systematically calculate the conduction band structure of biaxial tensile strain paralleled to (001),(110), and (111) crystal planes and uniaxial tensile strain paralleled to [001], [110], and [111] crystal direction in Ge_1-xSn_x alloys based on the deformation potential theory. Results indicate that the descent speed in the Γ valley is faster than that in the L valley in the case of biaxial tensile strain paralleled to (001) and (110) crystal planes and uniaxial tensile strain paralleled to [001] crystal direction in Ge_1-xSn_x. However, the descent speed in the L valley is faster than that in the Γ valley in the case of biaxial tensile strain paralleled to (111) crystal plane and uniaxial tensile strain paralleled to [110] and [111] crystal directions in Ge_1-xSn_x. The strategy of tuning Ge_1-xSn_x alloy into a direct band gap material is proposed for reducing Sn composition based on biaxial tensile strain paralleled to (001) and (110) crystal planes and uniaxial tensile strain paralleled to [001] crystal direction in Ge_1-xSn_x alloy which will provide references for the experimental preparation and device simulation.