Abstract: The bus voltage will drop when the motor starts. Different starting modes will lead to different amplitude of bus voltage drop under different power grids. According to the ratio of power grid capacity and motor capacity,it selects the appropriate starting mode,so as not to affect other electrical equipment under the same bus. In the specific application scenario of blast furnace blower,it starts from the restriction of power grid on soft starter. This paper gives the method of how to choose which soft starter economically under different power grid capacity,so as to improve the technical and economic ratio of engineering project.