We propose a polarization insensitive, flexible ultra-broadband terahertz (THz) metamaterial absorber. It consists of a chromium composite resonator on the top, a polyimide (PI) dielectric layer in the middle, and a chromium substrate. The simulation results show that the absorption achieves more than 90% ultra-wideband absorption in the range of 1.92-4.34 THz. The broadband absorption is produced by the combination of electric dipole resonance and magnetic resonance, as well as impedance matching with free space. Due to the rotational symmetry of the unit structure, the absorber is insensitive to polarization of the THz wave and has a larger range of incident angles. The total thickness of the absorber is only 13.4 µm, showing highly flexible and excellent high-temperature resistance characteristics. Therefore, it has potential applications in THz wave stealth and electromagnetic shielding.