Quick response and high sensitivity are equally important for the practical application of gas sensors. In this study, we introduced polyoxometalates (POMs) into a classical ternary metal oxide CuBi₂O₄ for the first time by convenient electrospinning and prepared a group of heater-type gas sensors by using CuBi₂O₄/PW₁₂ composite nanofibers. The sensor performances to formaldehyde gas were explored. The results proved that the gas-sensing response of the CuBi₂O₄/PW₁₂ (PW₁₂ = H₃PW₁₂O₄₀·xH₂O) composite sensor to 100 ppm formaldehyde gas was increased to 6.68, which was 3.92 times greater than the sensing capacity of the pure CuBi₂O₄ sensor. Simultaneously, the sensor exhibited a highly rapid response/recovery time of only 1/2 s, which is significantly faster than performances of CuBi₂O₄ gas sensors reported in the past literature. The cause of the improvement of the sensing performance was assessed via mechanism study, and it was found that the introduction of PW₁₂ into the sensor contributed to its enhanced performance. It was found that PW₁₂, as an electron acceptor, increased carrier mobility and reduced electron–hole recombination, thus contributing to enhanced gas-sensing properties. Moreover, other gas-sensing parameters such as selectivity, humidity resistance, repeatability, long-term stability were investigated. Hence, this study contributed to the literature on the development of polyoxometalates-based gas sensors.