We propose an ultra-thin multiband terahertz metamaterial absorber, whose thickness is only 3.8μm. Simulation results show that we can get four narrow absorption peaks with near-perfect absorption in the 4.5 THz-6.0 Thz frequency range. The resonance absorption mechanism is interpreted by the electromagnetic field energy distributions at resonance frequency. Moreover, we also analyze the sensing performances of the absorber in the refractive index and the thickness of the analyte. The refractive index and thickness sensitivities of the sensor are 0.471THz/RIU, 36.594THz/RIU and the FOMs are 8.887RIU -1 , 938.308RIU -1 , respectively. The absorber has potential applications in photodetector, multi-spectral imaging and biosensors.