The directional solidification of Ni-Cu alloy was simulated numerically based on a phase filed method, and the effects of pulling speed and thermal gradient on the interface shape in a forced flow/free flow were studied comparatively. The results indicate that the growth of the solidification interface incline towards the upstream direction in the forced flow. With the forced flow velocity increasing, the cells become bigger, the inclination depth of grooves is increased gradually and the depth difference becomes more obvious. With the increase in the pulling speed, the morphological evolution mode of directional solidification is planar interface→deep cells→ shallow cellular→ planar interface, while with the increasing thermal gradients the morphological evolution mode is deep cells → shallow cellular → planar interface. In addition, the forced flow has a great effect on the primary spacing of cells formed by solidification