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 Cu₃Mo₂O₉/MoO₃ composite gas sensing material is prepared by introducing Cu₃Mo₂O₉ nanoparticles on the surface of MoO₃ nanobelts. The results of SEM and TEM images show that the Cu₃Mo₂O₉ nanoparticles are uniformly dispersed. Then, the TMA sensing performance of a resistance-type gas sensor based the prepared Cu₃Mo₂O₉/MoO₃ composite 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 Cu₃Mo₂O₉/MoO₃ composite 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 MoO₃ nanoribbons to Cu₃Mo₂O₉, and the Fermi level reaches equilibrium. This process results in the formation of an electron loss layer underneath MoO₃, 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 MoO₃ to Cu₃Mo₂O₉. 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 Cu₃Mo₂O₉/MoO₃ nanocomposite can be explained by the enhancement of gas adsorption and diffusion: MoO₃ nanoribbons as a skeleton can effectively disperse Cu₃Mo₂O₉ particles and increase the adsorption capacity of gas molecules. And the enhanced response of Cu₃Mo₂O₉/MoO₃ may be due to the good catalytic effect of Cu₃Mo₂O₉, which is conducive to oxygen adsorption. This work provides a new strategy for preparing high-performance MoO₃-based gas sensing materials.