Without taking the Gaussian formula as a premise，the flux of the vector field passing through the infinitesimal closed surface is calculated from the beginning，so as to establish the divergence concept and derive the formula for calculating the divergence in the space rectangular coordinate system. Giving a beautiful area division formula，it can generally calculate the volume of the space area enclosed by the surface according to the shape and size of the closed surface. It is pointed out that people often fall into misunderstanding when they calculate the formula of the divergence by calculating the flux of the vector field through the surface of the infinitesimal hexahedron.