Using Tokuda's improved linear combination operator method and variational technique, the expression of the polaron effective mass in a parabolic quantum well is derived. Due to the spin-orbit interaction, the effective mass ratio of polaron splits into two branches. The dependence of the effective mass ratio on the vibration frequency, the spin-orbit coupling parameter, and the velocity is discussed by numerical calculation in the presence and absence of phonon. The effective mass ratio of polaron is an increasing function of vibration frequency. The absolute value of the spin splitting effective mass ratio increases with the increase in the spin-orbit coupling parameter and decreases with the increase in velocity. Due to the heavy hole characteristic of spin-orbit interaction, the spin splitting effective mass ratio is negative. The effective mass ratio is larger in the presence of a phonon than in the absence of a phonon, and the effective mass ratio splitting distance is independent of the phonon. topics: parabolic quantum well, effective mass spin splitting, vibration frequency, heavy hole characteristic