In our theoretical approach, the relaxation mechanism of the quasi-positronium (qPs) in alkali halides has been studied. It can be shown that the electronic cloud polarization in the medium results in the screening effect on the Coulomb potential of electron-positron pairs, so as to lead to the relaxation phenomena. Because the mean distance between the electron and positron in qPs is of the order of the lattice constant in alkali halide crystal, the Toyozawa's Hamiltonian in which only long range interaction between a charged particle and the medium is considered, could not be fitted in with the case of the qPs. So we try to make "the short range force correction" to the interactive Hamiltonian by using phenomenological method. Then we can solve the total Hamiltonian into the effective Hamiltonian of the qPs by means of the method which had been used to treat the exciton by Ku Shih-wei. It is shown that the result of variational treatment of the effective Hamiltonian is in good agreement with the experimental data, so the relaxation mechanism of qPs can be interpreted quantitatively.