The existence of forced and natural melt convections in the liquid in front of dendritic tip will Change the thickness of diffusion layer, which can significantly influence the dendrite morphology. Using the cellular automaton method and considering the influences of melt convection and heat transfer on microstructure evolution, a new numerical model is established by coupling the Navier-Stokes equations, the heat transfer equation and the solute convection and diffusion equation. The influences of forced and natural melt convection on dendrite morphology evolution are investigated by this model. The three-dimensional simulations reproduce the equiaxed dendrite growth, and reveal the influence of convection on dendritic growth rate and tip radius. The effect of natural convection on columnar dendrite growth during directional solidification of NH4Cl-H2O solution is simulated via the model. Experimental validation of this solidification process is performed and compared with simulation results. The simulation results accord well with experimental measurements. Hence, the model is reliable and can be extended to the prediction of the behavior of dendrite growth in real alloys.