At present the main method of measuring contact resistance is bridge method or detecting voltage/current, the shortage is only suitable for offline applications. A new method of online measuring contact resistance based on the transient thermal circuit model is raised. Firstly the theoretical physical model and the radial transient thermal circuit model of electric contact components are established, the thermal circuit response equation is concluded based on theory analysis. Then the axial heat transfer numerical analysis model is established. Finally the contact resistance model considering temperature and current is derived from the theory analysis, a method of measuring contact resistance by using the curve of thermal time constant is raised. A lot of theoretical analysis/calculation and finite element analysis of ANSYS simulation results show that the method can accurately obtain the value of the contact resistance, which provides theoretical support for on-line monitoring of contact resistance and thermal defect analysis of electric power equipment.