Abstract: The current flowing over and shielding in crack prevention using electro-magnetic heat effect is studied with numerical simulation method. In the analysis, the thermal-electrical coupling theory is applied. The numerical calculation shows that owing to the existing of crack, the current concentration is so remarkable that the crack tip is melted and a welded joint is formed. Consequently, crack propogation is prevented. When the same current is acted in the conductor with more than one cracks, the current flowing over and shielding effect is not remarkable, and the temperature and the temperature gradient at crack tip are mainly affected by the length dimension of cracks. With the change of the crack length, the temperature and the temperature gradient at the crack tip change markedly. The results of quantitative analysis of temperature and the temperature gradient are given in the paper.