In order to determine a cross-sectional profile of a clad bead in coaxial laser cladding, its formation mechanism is investigated theoretically and experimentally. In laser cladding, every point at the back edge of a melt pool is contributed to a cross-sectional profile of the clad bead to be formed, and points at the same pool edge but on different cross sections are located at different cross-sectional profiles of the clad bead. A cross-sectional profile of a clad bead is composed of points of intersection between the cross section and a series of pool edges. Model of the cross-sectional clad profile in single-pass coaxial laser cladding is developed. A 500W CO2 laser is used in the experiment. The experimental result agrees well with the calculated cross-sectional clad profile.