We explore the time-evolution law of the optical field of degenerate parametric amplifier (DPA) in dissipative channel. It turns out that its density operator at initial time ρ 0 = A exp(E ∗ a †2) exp(a † alnλ) exp(E a 2) evolves into ρ ( t ) = A λ ′ $\rho (t)= \frac {A}{\lambda ^{\prime }}$ exp E ∗ e − 2 κt a † 2 λ ′ 2 exp a † a ln [ λ − ( λ 2 − 4 | E | 2 ) T ] e − 2 κt λ ′ 2 exp E e − 2 κt a 2 λ ′ 2 , $\exp \left (\frac {E^{\ast }e^{-2\kappa t}a^{\dag 2}}{ \lambda ^{\prime 2}}\right )\exp \left \{a^{\dag }a\ln \frac {[\lambda -(\lambda ^{2}-4|E|^{2})T]e^{-2\kappa t}}{\lambda ^{\prime 2}}\right \} \exp \left (\frac { Ee^{-2\kappa t}a^{2}}{\lambda ^{\prime 2}}\right ),$ where κ is the damping constant of the channel, T = 1 − e −2κt , and λ ′ ≡ ( 1 − λT ) 2 − 4 | E | 2 T 2 . $\lambda ^{\prime }\equiv \sqrt {(1-\lambda T)^{2}-4|E|^{2}T^{2}}.$ We employ the method of integration (or summation) within an ordered (normally ordered or antinormally ordered) of operators to overcome the obstacles in the process of calculation.