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By means of experiments, the damping characteristics of the cast iron ground surfaces in contact normal was studied. It was found that the formulation of complex damping was a better representation of the test results and rules than the formulation of complex stiffness. Firstly, the relationship between contact pressure and deformation contact layer subjected to a harmonic excitation was studied and the complex damping method was used to establish the contact dynamic equation of steady state. The phase that normal contact deformation lags behind exciting force was used to reflect contact damping. Secondly, effect of normal pre-loaded pressure, amplitude of exciting force, and exciting frequency on the lagging phase were investigated. The results showed that the phase varies between 1.5° and 55° when pre-loaded pressure was from 0.205 MPa to 1.68 MPa, and amplitude of exciting force was from 22.7 kPa to 296 kPa and exciting frequency was from 0.25 Hz to 128 Hz. The relationship between the phase and pre load pressure was linear, and with pre-loaded pressure increased, the phase decreased. The relationship between the phase and exciting frequency was almost linear when exciting frequency was relatively high, and with frequency increased, the phase increased. It was also found that the relationship between the phase and amplitude of exciting force was non-monotonic. When the amplitude of exciting force was relatively small, the phase had a maximum value. In other words, only under modest amplitude of exciting force, the contact damping had a maximum value. 