An ultrawideband, polarization-insensitive, metamaterial absorber for oblique angle of incidence is presented using characteristic mode analysis. The absorber consists of conductive meander square loops and symmetric bent metallic strips, which are embedded with lumped resistors. With the aid of modal currents and modal weighting coefficients, the positions of the lumped resistors are determined. After that, the equivalent circuit (EC) model and admittance formula are proposed and analyzed to further understand the working principle and ultrawide bandwidth. The proposed absorber measures an absorption bandwidth of 4.3–26.5 GHz (144.1% in fractional bandwidth) for 90% absorptivity under normal incidence. At the oblique angle of incidence of 45°, the bandwidth of 90% absorptivity is still 5.1–21.3 GHz (122.72%) for transverse electric (TE) polarization, and 6.8–29.5 GHz (125.07%) for transverse magnetic (TM) polarization. The good agreement among simulation, measurement, and EC calculation demonstrates the validity of the proposed method and indicates that the method can be applied to other microwave and optical frequency bands. The proposed metamaterial absorber can be widely applied in electromagnetic compatibility, electromagnetic interference, radar stealth, and biomedical detection.