The Hamiltonian of comb polymer nematic liquid crystal molecule is constructed by using the interaction model of nematic liquid crystal molecule proposed by Freiser and the mean field theory. The self-consistancy equations of order parameters are derived and the phase transitions from isotropics to uniaxial nematics, uniaxial to biaxial nematics are obtained. The results show that when the repulsion in coupling term of backbone and side chain getting greater, the nematic field of the stronger one between those of backbone and side chain getting stronger, the uniaxial-biaxial phase transition temperature will rise and the phase transition will gradually change from first order to second order phase transition.