Abstract: Based on intermolecular dispersion theory, this paper analyzes the microscopic mechanism of magnetic viscosity decreasing in quantum mechanics and statistical mechanics, and summarizes the occurrence conditions of inverse effects. According to statistical mechanics, the electrons in the molecule are in Boltzmann distribution, and based on quantum mechanics we can derive that the intermolecular dispersion energy will increase with the increase of magnetic field under thermal equilibrium conditions, and the greater the intermolecular dispersion energy is, the smaller the viscosity is, thus realizing a viscosity reduction; under non-thermal equilibrium conditions, the intermolecular dispersion energy will decrease with the increase of the magnetic field, thus inverse effects occur. Because the crude oil viscosity mainly depends on the understanding of strength of its mesh structure of wax crystal, an alternating magnetic field is added to improve the viscosity decreasing effect and to prolong the time to be needed for the viscosity restoration of crude oil after magnetic treatment.