The assymmetrical dendritic growth and the effect of flow velocity on the temperature distribution and laminar flow boundary layer of both facing-flow side and against-flow side has been investigated numerically based on the phase-field model which incorporates both fluid flow and thermal noise, as well as reasonably high efficiency 3DAADCR. The computed results indicate that, the forced flow causes the dissymmetry of the distribution of the temperature distribution and laminar flow boundary layer of both facing-flow side and against-flow side, leading to the different undercooling on either sides, as a result, the dendritic growth towards the flow and inclination relative to the dissipating heat direction appear. At the same time, the undercooling of facing-flow side is greater than that of against-flow side, which is helpful to accelerate dendritic growth rate and makes the sidebranching steady growth, accordingly, resulting in assymmetrical sidebranching growth. With the increase of flow velocity, the greater the inclination, and the more obvious the assymmetrical degree.