The work aims to reveal the evolutions in microstructure of laser cladding Fe-based layers at different scanning speed, and then obtain an efficient way of controlling structure. Provided with the same F/V (powder feeding rate/laser beam scanning speed) and laser power, semiconductor laser was used to prepare laser cladding layers on the surface of nodular cast iron at different scanning speed. Microstructures and phases of the cladding layers were characterized with optical microscope, scanning electron microscope, energy dispersive spectroscopy and X-ray diffactometer. Hardness of the cladding layers were measured by Rockwell hardness tester. At scanning speed of 7 mm/s and dilution rate of 16%, microstructure of the cladding layers was mainly composed of martensite and a small amount of retained austenite, the grains were small, no crack existed and Rockwell hardness was 55.5HRC. At the scanning speed of 5 mm/s and dilution rate of 30%, grain morphology of the cladding layers became more coarser, more columnar crystals and equiaxed crystals were obtained, and content of retained austenite increased and content of martensite decreased significantly, but the cladding layer presented bigger crack susceptibility and Rockwell hardness decreased to 21.7HRC. Scanning speed plays a great role in preparation of laser cladding layers on the surface of nodular cast iron, and has great effects on solidification, phase composition, grain growth, crack susceptibility and hardness. Scanning speed can be adjusted to change content of mastensite and retained austenite in the cladding layers, obtain crack-free cladding layers of reasonable hardness, and further effectively control microstructures and mechanical properties of the layers.