The Fe-Cr-Si-P amorphous coatings are prepared on the 304L stainless steel surface by using the broad-band laser cladding technique. The microstructures and formation mechanism of these coatings are analyzed. The mathematical and physics models of laser cladding are established, and the change rules of temperature gradient and cooling rate of molten pool along the depth direction are obtained. The results show that the microstructures of the coating are planar crystalline and epitaxial growth dendrites in the bonding zone, amorphous character in the middle zone and fine equiaxed crystalline on the surface. During the solidification process, the temperature gradient from the bottom to the surface for the molten pool decreases and the cooling rate increases gradually. Based on the rapid solidification theory, the model of the relationship between the coating microstructure character and the shape control factor as well as that between the coating microstructure character and the cooling rate is established.