The variation in the grain morphology and solute concentration distribution has the noticeable impact on the performance of welded joints in laser welding. In this paper, the grain growth and transition process at tip position is calculated with acquired transient solidification conditions by the developed macro-micro coupled model including the macroscopic heat transfer and fluid flow model, transient solidification conditions model and microscopic phase field model. The validity of the model is confirmed by comparing the experimental and calculated results. The dynamic evolution behaviors of grain growth and transition process are analyzed in detail. It is found that the grain morphology is transformed from columnar grain to equiaxed grain during the molten pool solidification process and the equiaxed grains which nucleate in the front of columnar grains suppress the growth of columnar grains. The secondary dendrites of some columnar grains are developed in columnar grain region and the number of equiaxed grains increases in transition region from columnar grain to equiaxed grain. In the equiaxed grain region, the solute concentration distribution is more uniform and the grain boundary segregation is lower than those in the columnar grain region. The analysis results of grain growth and transition behaviors can provide a guidance for improving the welding quality.