Complicated morphologies of directional solidification structures attract a lot of theoretical studies and commercial uses. As known, the boundary heat flux has an important significance to the microstructures of directional solidification. In this article, the interface evolution of directional solidification with different boundary heat fluxes is discussed. In this study, only one interface has heat flow, and Neumann boundary conditions are imposed at the other three interfaces. From the calculated results, it is found that different heat fluxes cause different microstructures in the directional solidification. When the heat flux equal to 18 W/cm(2), the growth of lengthways side branches is accelerated and the growth of transverse side branches is restrained. At the same time, there is dendritic remelting in the calculating domain. When the heat flux equal to 36 W/cm(2), the growth of the transverse side branches and the growth of the lengthways side branches compete with each other. When the heat flux equal to 90 or 180 W/cm(2), the growth of transverse side branches absolutely dominates. The temperature field of dendritic growth is also analyzed and the relation between heat flux and temperature field is found.