CdMoO4 nano-octahedrons were successfully prepared using a one-pot hydrothermal method, and then decorated with Ag nanoparticles (Ag NPs). Tests of the sensing performances confirmed the superior sensing properties of the Ag@CdMoO4 sensor. The sensor exhibited the strongest response value of 140.1 at 260 °C and a shorter response/recovery times (15 s/8 s) for 100 ppm triethylamine (TEA) gas compared with other detected gases. The detection limit for TEA gas was 10.6 ppb. The sensor also exhibited excellent stability. Favorable octahedrons-like nanostructure, higher relative percentage of the Oads, high catalytic activity of metal elements, and electron sensitization effect of Ag NPs jointly improved gas-sensing performances. The Ag-decorated CdMoO4 nano-octahedron sensor has great potential for the accurate and rapid detection of TEA gas with a strong sensing response.