The influence of phonon dispersion on properties of the magnetopolaron in a polar crystal was studied by using the linear combination operator and unitary transformation method. Taking account of the longitudinal optical (LO) phonon dispersion in a parabolic approximation, the ground state energy, self-trapping energy and ground state Landau energy of the magnetopolaron in a polar crystal as a function of coefficient of the phonon dispersion, cyclotron resonance frequency and electron-LO-phonon coupling constant were obtained. Numerical calculation results show that the ground state energy decreases with increasing the coefficient of the phonon dispersion and electron-LO-phonon coupling constant. The self-trapping energy increases with increasing the coefficient of phonon dispersion and the electron-LO-phonon coupling constant. The ground state Landau energy increases to a maximum and then fall with increasing the electron-LO-phonon coupling constant, and increases with increasing the coefficient of phonon dispersion.