A novel ultra-thin microwave metamaterial absorber design is proposed and experimentally demonstrated. The metamaterial absorber consists of three metallic structural layers and two dielectric-material layers. Two identical metallic circular rings are used as the double composite resonant structural units. Compared to the traditional resonant unit with a single-layered pattern, this new design has an efficient enhancement in the absorptivity of electromagnetic wave as well as an obvious reduction in the thickness of the whole structure. The electromagnetic simulation based on the finite element method is employed to simulate and analyze the distributions of inner spatial electromagnetic field and surface current of this absorber, and the absorption mechanism of electromagnetic waves is expounded. The simulated and experimental results both confirm this absorber is polarization-independent and stable for the wide oblique angle incidence. This ultra-thin metamaterial absorber with a simplified structure and a low duty cycle has potential applications in many practical areas such as electromagnetic window shielding.