A terahertz polarization-insensitive broadband absorber is designed. Its absorption spectrum's full width at half maximum (FWHM) is 2.29 THz, and the bandwidth is about 1.65 THz when the absorptivity is more than 97%. Further, the absorptivity becomes 99.9% at some resonant frequencies, indicating that it can achieve a perfect absorptivity of approximately 100%. The absorber stacks three layers of periodic units exhibiting the same structure but different sizes, and it can effectively expand the bandwidth. This study analyzes the absorption mechanism and discusses the influences of different parameters on its performance. The simulation results denote that the absorption characteristics of the absorber are polarization-independent and that it can maintain a wide bandwidth over a certain incident angle range. This polarization-insensitive broadband terahertz absorber that exhibits high absorptivity has significant research value in terahertz imaging, terahertz wave detection, electromagnetic stealth, and other such applications.