Triethylamine (TEA) is a crucial raw material in industrial processes. However, exposure to this gas poses significant health risks to humans. Therefore, the development of sensors that can effectively detect TEA with high performance is crucial. In this study, we synthesized Ag@ZnO hollow spheres using microwave hydrothermal and template methods. The Ag@ZnO microreactor structure was formed by decorating Ag particles on the inner wall of ZnO hollow spheres. This unique structure effectively extended the contact time between Ag particles and gas molecules and noteworthily enhanced the sensing properties. The gas sensing measurements demonstrated that the Ag@ZnO sensor exhibited ultrahigh selectivity and superior sensitivity to TEA. The response value to 100-ppm TEA was 763 at the optimal temperature of 200 °C, which was approximately 34 times higher than pure ZnO hollow spheres and three times higher than that of ZnO@Ag where Ag particles were loaded on the outer wall of ZnO hollow spheres.