The flow and thermal fields within the infrared signature suppression(IRSS) device of a marine gas turbine exhaust system were investigated numerically. For the conjugate heat transfer within the device, an integrated solution method was adopted. The discretion equation was derived for the control volume with conjugate heat transfer at its interface, with the radiation heat flux at the interface appearing as an additional source term. The solution method of velocities at the entrance of secondary flow was presented. The numerical results agreed well with corresponding experimental data, which are the basis of the prediction of infrared radiation distribution of a marine gas turbine exhaust system.