A coupled active–passive double-cavity system with balanced effective gain and loss provides a powerful platform for sideband generation. In different input limits, we explore the gain-saturation nonlinearity of the active cavity with regard to sideband generation. Using a perturbation method, we give an explicit analytical expression for second-order sideband generation (SSG), showing a conversion efficiency of over 80% and pump powers as low as ∼μW. A remarkable mode-splitting is also observed during the SSG process in the unbroken parity–time () symmetry phase. With change in the system parameters, our numerical method shows that high-order sideband generation (HSG) in the unbroken phase is controllable, including increasing its amplitude and changing its order number, especially when switching HSG between perturbative and non-perturbative regimes. In the broken phase, field localization effect leads to a stronger output spectral intensity than that in the unbroken phase.