Master equations of density operators in the diffusion process can be concisely solved by the technique of integration within an ordered product of operators and virtue of thermo- entangled state representation. The evolution formula of the field density operator which is chaotic field initially is got. The results show that the chaotic field maintains its original properties, but its average number of photons increases linearly in the diffusion process. The photon number fluctuation increases in the form of a quadratic function with diffusion time.