Recidual magnetic method is an important method in lightning calamity investigation which can determine whether lightning accident occurred, but the standard only provided the reference data of the recidual magnetic, the generality of the data must be considered again. In order to explore the regularity of the recidual magnetic of the iron casting, it chose some samples to make impulse test by simulating the occurrence of the lightning current, then measured the recidual magnetic and showed that the main factors which influence the recidual magnetic include the material, the size, the position of the iron casting, the position of the measurement, the amplitude of the lightning current, the impact condition, and the impact times. The results show that the data from 0.5mT to 1.5mT can also be the basis. And because of the cumulative effect by the iron casting, the recidual magnetic tends to be saturated after 5 times of the impact. It also provides a theoretical basis for lightning calamity investigation.