Under the condition that there is only solid-liguid transition in a metallic material, we found the laser power density for melting the material. Through analysis of the similarity of some behaviors of solid and ciquid states, we obtain the absorptivity of laser by the metal, and indicate the impossibility to directly calculate the transition rate of the metal. However, we can obtain the transition rate under the irradiation of laser, and find the relationship of the transition rate with the intensity and the frequency of the laser.