The lateral impedance of a tapered pile embedded in saturated soil medium is theoretically investigated with the consideration of soil disturbances in radial direction. To process the variation of pile radius and the disturbance of soil，the soil-pile system is discretized into axial thin layers and the soil layers are further discretized into radial annular layers. A lateral complex transfer model is developed to determine the reaction of the radially disturbed saturated soil layer to the pile segment based on the Biot’s theory and plane-strain assumption. The differential equation for the transverse vibration of pile segment in each soil layer is established based on the Timoshenko beam theory. The semi-analytical solution of the lateral impedance at the head of the tapered pile is derived by the transfer matrix method. Parametric analyses are performed to investigate the influence of the soil and pile properties on the lateral impedance. The results are concluded as follows：Increasing tapered angle can increase the impedance of tapered pile under the low excitation frequency. The impedance of the pile with a larger taped angle shows a stronger frequency-dependence with the increase of frequency. The weakening of soil around the pile reduces the lateral impedance of the tapered pile under low frequency. Weakening effect increases the resonance amplitude of the impedance with the increase of frequency. The soil medium with high permeability such as sandy and gravelly saturated soil has noticeable influence on the lateral impedance. It is necessary to consider the rotational inertia and shear deformation for tapered piles with small slenderness ratio and high excitation frequency.