Some problems still exist in the directionally solidified superalloy, such as coarse dendritic structure, thick second phase and composition segregation. Many efforts have been made to enhance the overall performance of superalloy by improving the components and processes. However, the higher alloying degree not only increases the solidification segregation, but also increases the tendency of precipitating σ-phases, Laves-phases, μ-phases and other harmful phases. Therefore, it is crucial to improve the processes in order to enhance the performance of superalloy. It was known that the magnetic field effect is very obvious during the melt solidification process, which has been realized in the directional solidification of silicon, while the research on the magnetic field effect during the superalloy solidification process is few. So it is necessary to research the effect of magnetic field on directionally solidified superalloy. In this paper, the influence of horizontal magnetic field (≦0.8 T) on the microstructures of the superalloy DZ417G during directional solidification was investigated. It was found that the magnetic field can induce the appearance of freckle-like macrosegregation and the decrease of the primary dendrite arm spacing at a low growing speed, however, with the increase of the growth speed, the effects become weak. Based on the thermoelectric magnetic convection (TEMC) induced by the magnetic field during solidification and the effect of the convection on the microstructure, the above experimental results were discussed and analyzed.