Base on successful solution of the problems in judging whether the surface elastic term K13 of nematic liquid crystal exists or not, the equilibrium equation and boundary condition of the director have been obtained by the methods of analytical derivation and numerical calculation. The influence of the surface elastic energy term K13 on the properties of Fréedericksz transition of weak anchoring nematic liquid crystal cell under a magnetic field has been investigated in detail. The results show that the existence of K13 has influence on the free energy of the liquid crystal system so great as to change the property of the transition and induce a first-order Fréedericksz transition. The conditions for this transition are obtained. They depend on the surface elastic energy term K13 besides the structure and material of the system. The method of judging whether K13 term exists or not by Fréedericksz transition has been given.