<p>Based on the numerical thermal-fluid coupling simulation and experiments of laser melting deposition(LMD), microstructure growth and evolution characteristics of the molten pool at different scanning speeds were investigated. Integrating experimental metallographic observations with quantitative analysis on shape control factor(<i>G</i>/<i>R</i>), cooling rate(<i>G</i>×<i>R</i>) and volume fraction of the equiaxed grains(<i>φ</i>) derived from the numerical temperature gradient(<i>G</i>) and solidification rate(<i>R</i>), the molten pool is divided into bottom, middle and top regions, and the grain type, grain size as well as grain transformation in each region are investigated separately and integrally. When increasing the scanning speed, the length of dendrites in the direction of temperature gradient decreases and it splits into smaller grain structures, and the grains are refined to be thinner and smaller, and therefore the microhardness of the deposition layer is improved. Scanning speed is an effectively controllable process parameter in LMD that affects <i>G</i> and <i>R</i>, which are the critical factors to be regulated for the targeted microstructures and mechanical properties. By qualitative and quantitative discussion on solidification parameters, microstructure and microhardness, this integrated numerical and experimental analysis effectively relates process parameter, microstructure and mechanical property with intrinsic mechanisms.</p>