In this article, an ultra-wideband metamaterial absorber integrating flexibility and tunable intensity characteristics based on lossy stepped impedance resonator (SIR) is proposed. Compared with the uniform impedance resonator loop, the SIR loop can perform a lower fundamental resonant frequency while raising its first-order resonant frequency by adjusting characteristic impedances of its different sections, thus facilitating a wider absorption bandwidth. The voltage-controlled PIN diode is then properly loaded for the lossy characteristics of the SIR, thereby simultaneously enabling ultra-wideband and tunable absorption intensity. Detailed analysis of a quarter-wavelength lossy SIR is conducted to reveal the resonant mode characteristics. Moreover, the equivalent circuit model (ECM) of the lossy-SIR-based absorber is developed to explain the operating principle and facilitate our discussion on the parametric effects. Finally, the proposed absorber is fabricated by the flexible printed circuit process and measured to verify the design methodology. The measured effective absorption bandwidth is 4.3-17.8 GHz (122.1%) for transverse electric (TE) polarization, and 5.2-17.6 GHz (108.7%) for transverse magnetic (TM) polarization. The proposed absorber has the unique advantages of ultra-wideband absorption, wide tunable absorption intensity, and quasi-single-layer flexible structure, simultaneously, which is of great significance for application in object conformality and dynamic radar cross-section (RCS) reduction.