Abstract: Quantitative analysis of cracks is the critical issue for eddy current thermography to make a breakthrough progress.The simulation models of surface cracks coupling Joule heating with heat transfer have been established via COMSOL software.Sequential thermal images have been obtained during the heating and cooling periods of the test process.By the aid of Matlab toolbox, the surface temperature distributions on the tip and centre point of the cracks have been extracted.Lagrange interpolation has been employed to quantitate crack depth.The crack depth is calculated by Lagrange interpolation with the lengths and depths of cracks.The proposed method has been validated by the simulation results.