Based on the finite element ABAQUS code, a sequentially coupled thermal stress procedure was developed to calculate the distribution of temperature and thermal stress field of coked HP40Nb tube, and the critical depth of coking layer was predicted when the decoking measurement should be conducted. The results show that the thermal diffusion property of HP40Nb alloy deteriorates sharply with coking layer increasing. The maximum circumferential stress is along the crosssection of coking layer and HP40Nb alloy, and the stress increases initially and then decreases. Coking caused the ethylene pyrolysis furnace tube to bulge to the actual radial and rupture at local field. The critical depth of coking layer was predicted to be 2.3 mm when the decoking measurement had to be executed.