Based on the infinite operator-sum solution to the master equation dρ dt = −κ(α † αρ − α † ρα − αρα † + ραα † ) describing the diffusion channel we explore time evolution law of Wigner function of quantum states in this channel. By virtue of the technique of integration within an ordered product of operators we demonstrate that the initial Wignerwhere κ is the diffusion constant. Then the time evolution of any density operator ρ's Wigner function can be calculated by directly tracing out T r ρ (α, α * , t) .