The influence of phonon dispersion on properties of the average optical phonon number of weak-coupling polaron in a parabolic quantum dot was studied by using the linear-combination operator and unitary transformation method. Taking account of the longitudinal optical (LO) phonons dispersion in a parabolic approximation, the ground state energy and average number of virtual phonons around the electron as a function of the coefficient of the phonon dispersion were obtained. Numerical calculation results show that the ground state energy decreases with increasing the coefficient of the phonon dispersion, and average number of virtual phonons around the electron increases with increasing the coefficient of phonon dispersion.