The physical and chemical properties of CdIn2S4 can be effectively regulated by changing the composition and phase structure to present a rich diversity, making it a potential new gas sensing material. However, CdIn2S4 has the characteristics of low carrier mobility and difficult separation of electrons and holes, which greatly hinders its development in the fields of gas sensing and catalysis. In this work, a sulfur-vacancy-rich CdS/CdIn2S4 heterojunction composite was constructed by a one-step hydrothermal method for gas sensing, and the built-in electric field was used to effectively promote the separation and transfer of charges. Moreover, the introduction of sulfur vacancies increases the content of chemisorbed oxygen, which provides more reactive sites for the target gas, thereby realizing the improvement of gas sensing performance. Compared with the CdIn2S4 sensor, the CdS/CdIn2S4 sensor exhibited higher response to triethylamine (10 ppm-32.5), faster response/recovery time (3 s/256 s) and achieved a lower limit of detection at the ppb level (500 ppb-5.2). This work provides a new solution for real-time monitoring of low-concentration triethylamine gas.