When the superthermal electrons produced by ultrashort and ultrahigh laser interacion with solid target tranport in the target, the Kα X-rays will irradiate from the target. According to classical scaling law, superthermal electrons' temprature in the normal direction can be got. Using Monte Carlo method, the tranportation of electrons in the solid target is researched. We simulate the Kα yield and spatial distribution for different target thickness, also simulate the Kα conversion efficiency versus temprature. Simulation result accords with experimental results. We conclude that, at a given temprature, the Kα yield will reach saturation and the spatial distribution will become more nonisotropic when the target thickness increases. There exists an optimial temprature for Kα X-rays conversion efficiency.