Mid-infrared thermal detectors have very important applications in the aerospace and military fields. However, due to the low heat transfer efficiency and slow response time, their application has been greatly restricted. Here, we theoretically demonstrate a dual-band perfect absorber for a mid-infrared detector based on a dielectric metal metasurface, and the optical and thermal properties are analyzed in detail. Simulation results show that the two narrow absorption peaks, corresponding to the absorption value of 97.5% at λ=6.142μm with λFWHM≈40nm and 99.7% at λ=7.795μm with λFWHM≈80nm, respectively, are achieved, and their different dependences on the structural parameters have been studied. A thermal detector at room temperature with total response time within 1.3 ms for dual-band and 0.4 ms for single-band is realized when the incident light flux is 1.0W/cm2 for an average temperature increase of ΔT≈1.0K. Our study offers a promising approach for designing a narrowband mid-infrared perfect absorber and a high-performance photodetector in nano-integrated photonics.