Abstract: The theory of defect penetrant was detailed analyzed, the effects of the penetrant materials, defect types, air pressure and defect position on the inspection sensitivity were discussed. According to the hypothesis: during the rising of liquid pillar, the density of the liquid is disregarded, it infers the parameter of dynamic penetrant by way of fluid dynamic method; It assumes that the temperature and mass of the hermetical gas keep unchanging in the process of the crack model being penetrated, and infers the penetrating volume of the penetrant liquid in the crack model based on laws of thermodynamics.