Using Ritz method in obtaining the surface order parameters,we obtained a phase transition from Bc,a biaxial nematic along the film normal,to phase B a,a biaxial phase along the axis in the film plane.The result shows t hat as the biaxial part of the anchoring strength increases,the mechanism of the occurrence of Ba changes from temperature decreasing to surface anch oring.Within a certain range of uniaxial anchoring strength α,increasing α wil l make Ba occur at a higher temperature while further increasing of α will make the liquid crystal reenter Bc from Ba.