Formaldehyde is a hazardous volatile organic compound (VOC) widely present in indoor and industrial environments, and its reliable detection is essential for environmental monitoring and health protection. Two-dimensional (2D) materials have attracted increasing attention for gas sensing applications due to their large surface area and surface-sensitive charge transport. Among them, MXenes exhibit high electrical conductivity and rich surface chemistry, making them attractive sensing materials. However, the limited electrical signal modulation in pristine MXene constrains its sensing performance. Constructing MXene/Transition metal dichalcogenides (TMDs) heterostructure provides an effective approach to regulate interfacial charge distribution and thereby enhances gas sensing performances. In this work, 2D WSe₂@Ti₃C₂T_x heterostructure is rationally designed and employed for gas sensor development. The limit of detection (LOD) of the gas sensor for formaldehyde is 143 ppb. The heterostructure serves as a critical bridge between gas adsorption and electronic transport, highlighting the potential of 2D MXene/TMDs heterostructure for ppb-level formaldehyde sensing.