Metamaterials have many exotic optical properties due to the interaction between the electromagnetic wave and the nanostructures. Metamaterials (PAMs) with metal-dielectric-metal structure have perfect absorption characteristics. The optical parameters of a cross-shaped absorber in infrared band are retrieved by the S-parameters method. The mechanism of resonant absorption and optical tunability are studied. The experiments and simulations results indicate that the size of cross-shaped absorber can tune its effective optical parameters and can effectively increase its absorptivity on the conditions that the effective impedance matches well to the incident medium, and the imaginary part of the effective refractive index approaches as great value as possible. The absorptivity of optimized cross-shaped absorber is greater than 95% and maximum absorption of 80% is experimentally obtained in infrared region. The resonance peaks of the absorption spectrum depends on the length of the cross arm and thickness of the dielectric materials, while the width of the cross arm plays a critical role to the maximum absorption value. The characteristics of perfect absorption and spectral tenability of corss-shaped absorber make it promising in many applications, including infrared detection, spectral imaging and so on.