阐述了矢量场穿过2 阶无穷小平面的通量虽然也是2 阶无穷小量，但特殊情况下要求计算精度必须达到3 阶无穷小量，为此目的需要把2 阶无穷小平面分成无穷多个4 阶无穷小面元来计算通量，并且给出如此计算所得结果遵循的规律———3 阶无穷小精度下矢量场穿过2 阶无穷小平面的通量定理.最后介绍了这个定理在散度理论和电磁场法向边值关系理论中的应用.

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.