Abstract. Aiming at the requirement of workpiece integrity for parts processing line, on-line detection using inductive heating thermography for the moving workpieces on the assembly line is studied. In this paper, the detection mechanism of pulsed eddy current thermography for moving workpieces defects is analysed. A two-dimensional model of a magnetic material (45 steel), on which there is a crack parallel to the coil is established by the finite element software named COMSOL 5.2. By analysing the changes of the temperature curves, normalized curves and the temperature difference curves, the optimal detection area for parallel cracks is proposed. The consistency of the conclusions is verified by the experimental platform. The paper can provide a theoretical guidance for quantitative detection using eddy current thermography.