Aquatic products contain an incredibly high nutritional value for the human body and gradually become indispensable ingredients on the Chinese table. Trimethylamine (TMA) from the deterioration of aquatic products can serve as an indicator to measure fish freshness. It is a challenge to develop an instant, fast, convenient, and efficient gas sensor for fish freshness. In this study, a novel Cu ₃Mo ₂O ₉/MoO ₃composite gas sensing material is prepared by introducing Cu ₃Mo ₂O ₉nanoparticles on the surface of MoO ₃nanobelts. The results of SEM and TEM images show that the Cu ₃Mo ₂O ₉nanoparticles are uniformly dispersed. Then, the TMA sensing performance of a resistance-type gas sensor based the prepared Cu ₃Mo ₂O ₉/MoO ₃composite is tested at optimal operating temperature (240 °C). the results show that the sensor possesses good response (13.9) at low concentration (5×10 ^–6), with excellent low detection limit (2×10 ^–7). The response time is also significantly shortened. The high sensing performance of Cu ₃Mo ₂O ₉/MoO ₃composite is attributed to the heterojunction interface, which promotes the separation of electrons from holes through its strong oxygen adsorption and catalytic effect. This significantly improves the electron transport properties and gas sensing characteristics of the composite material. Electrons flow from MoO ₃nanoribbons to Cu ₃Mo ₂O ₉, and the Fermi level reaches equilibrium. This process results in the formation of an electron loss layer underneath MoO ₃, and the charge transfer channel narrows, which is consistent with previous result. When trimethylamine dissociates on the nanoribbons to release electrons, the balance of the fermi lever is disrupted, and electrons flow from MoO ₃to Cu ₃Mo ₂O ₉. As a result, the charge transfer channel becomes thinner, resulting in resistance modulation and increased sensitivity. In addition, the enhancement of trimethylamine sensing performance of Cu ₃Mo ₂O ₉/MoO ₃nanocomposite can be explained by the enhancement of gas adsorption and diffusion: MoO ₃nanoribbons as a skeleton can effectively disperse Cu ₃Mo ₂O ₉particles and increase the adsorption capacity of gas molecules. And the enhanced response of Cu ₃Mo ₂O ₉/MoO ₃may be due to the good catalytic effect of Cu ₃Mo ₂O ₉, which is conducive to oxygen adsorption. This work provides a new strategy for preparing high-performance MoO ₃-based gas sensing materials.