Based on the thermochromic phase transition characteristics of VO2, a tunable mid-infrared broadband absorber with a W/VO2 square nano-pillar array is designed. The influences of structural parameters on the absorption performances and the electromagnetic field intensity distributions within the structure, and the absorption characteristics of the absorber under different polarization states and incident angles are analyzed by the finite difference time domain method. The results show that the infrared light incident on the absorber is converted into heat and consumed when VO2 does not undergo the phase transition, and the average absorptivity of the absorber in 3-5 μm wavelength region is as high as 96.2% under the optimal structural parameters. In contrast, the absorber exhibits a strong reflection and inhibits light absorption when the phase of VO2 is changed into a metallic state, and the average absorptivity difference between high and low temperatures can reach 74.1%. The absorptivity of the absorber is less affected by the incident lasers with different polarization states and has wide-angle absorption characteristics, so the absorber is expected to be applied in the field of infrared intelligent optoelectronics.