The turboshaft-turbofan variable cycle engine represents an innovative concept in power device design. Its exhaust system must be structurally optimized for mode conversion while simultaneously incorporating infrared stealth capabilities. This paper proposes a two-dimensional exhaust system with variable modes， describes the structural characteristics under both the turboshaft and turbofan modes， and performs numerical investigations into the infrared radiation signature of the dual-modal exhaust system using techniques such as ejection， exhaust cone film-cooling， lobe mixer occlusion， and enhanced mixing. This paper also comparatively analyzes the influence of various parameters， including the width-to-height ratio of the two-dimensional nozzle outlet and the expansion angle of the lobe mixer in turbofan mode， the opening angle of the bypass valve in turboshaft mode， on the infrared radiation signature of the exhaust system. The comprehensive suppression measures implemented in the turbofan mode can enhance the backward infrared suppression performance by over 60%. The width-to-height ratio of the nozzle outlet has been increased from 2 to 4， which further improves the backward infrared suppression effect by 8%. However， at larger detection angles， there is a slight increase in infrared radiation. Increasing the internal expansion angle of the lobe enhances infrared suppression performance， whereas increasing the external expansion angle negatively impacts stealth capabilities. The influence of the bypass valve opening angle on infrared radiation signature in the turboshaft mode is complex. When both the ejection coefficient and the infrared suppression effect are maximized， it exists an optimal opening angle of 30°. Numerical simulation results confirm that the dual-modal exhaust system proposed in this paper not only fulfills the functional requirements but also demonstrates infrared suppression capability.