Expounded though the flux which vector field through the second order infinitesimal plane is also the second order dimensionless，but in particular cases demand the computational accuracy must achieve the third order dimensionless. For this purpose，it needs to divide the second order infinitesimal plane into infinitely many fourth order infinitesimal surface element to calculate the flux，and give the law that followed by the result that obtained through such calculation：The theorem of vector field through second order infinitesimal plane under third order infinitesimal accuracy. It also introduces this theorems application in the divergence theory and in the theory of electromagnetic field normal boundary value relations.