The exhaust system is the main infrared radiation source of an aero engine. Restraining the infrared signal is critical for the improvement of aircraft survivability. Addressing the optimization design of low infrared radiation for aircraft nozzles, this paper proposes a gradient calculation method for plume infrared radiation based on the adjoint approach. A coupled adjoint equation for infrared radiation and flow field is derived from the discrete transfer method and Navier–Stokes equation. The Malkmus statistical narrow-band model is constructed for gas transmissivity calculation. Variational expressions of Malkmus narrow-band model parameters with respect to flow field primitive variables and molar fractions are derived to enable efficient calculation of gas transmissivity variation. The infrared radiation and gradient calculation programs are validated using axisymmetric and serpentine nozzles. By comparing the gradient results from the adjoint method with finite difference results, this study demonstrates that the proposed infrared radiation gradient calculation method achieves high precision and efficiency, providing a foundation for gradient-based aerodynamic/infrared integrated optimization design.