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Conventional metasurfaces cannot adjust the electromagnetic responses once the structures are determined. In this paper, we numerically demonstrate a thermally switchable bifunctional metasurface based on the phase change material vanadium dioxide (VO2) applied to the mid-infrared band. The dual-layer ring resonator consists of an Ag split ring and a VO2 ring, which can achieve ultra-broadband polarization conversion and perfect absorption through the insulator-to-metal phase transition of VO2. When VO2 is in the insulating state, the designed metasurface acts as polarization converter with polarization conversion ratio of over 95% in the wavelength range of 2500 nm to 3500 nm. When VO2 is in the metallic state, the metasurface behaves as perfect absorber with over 90% absorptivity in the region from 2900 nm to 3700 nm. And the operating bandwidths of the two functions are highly overlapped. The ultra-broadband polarization conversion properties are mainly derived from the closed multiple electric and magnetic resonances. Single localized magnetic resonance results in a flat absorption spectrum. The proposed metasurface may provide new ideas for the design of tunable integrated optical devices.