To more comprehensively and precisely reproduce mechanical behaviors of frictional and describe properties of energy dissipation, a kind of Iwan model considering variable normal load is developed. In the normal direction, a nonlinear spring is used to describe normal load variation and intermittent separation. In the tangential direction, the original Iwan model is used to describe micro-slip and macro-slip behavior. The relationship between tangential restoring force and relative displacement is derived and the transition criteria among motion states of the model is given. A simplified friction oscillator under harmonic excitation is analyzed to study the effect of different parameters of excitation on the system responses. The proposed model degrades into the original Iwan model when the normal load keeps constant, which validates the model well. This model considering coupling of micro-slip tangential force and variable normal force more comprehensively, exactly reproduces contact interface and effect on the system response. The separation of interface not only reduces damping capability of friction system and makes the responses high, but also strengthens nonlinearity of the system, so it should be avoided in the practical engineering.