Dynamic stiffness is an important index in pile dynamic measurement and analysis. In practice it is important to analyze and identify the factors that influence the dynamic stiffness of pile foundation. To analyze the dynamic stiffness of the intact and defected piles, an analytical model of admittance response under vertical harmonic loads is developed and solved. In this model, two variable cross sections are considered. The measured results show that the model can be effectively used to calculate the values of resonance frequency and dynamic stiffness. Moreover, 513 bridge piles of the same type are measured using mechanical impedance method. The dynamic stiffness for all piles is analyzed statistically. It is shown as follows. (1) the supporting stiffness of pile tip and the pile length significantly effect dynamic stiffness; the dynamic stiffness of end-bearing pile decreases with its length, whereas that of pure friction piles increases with the pile length. If friction pile has a longer length, its dynamic stiffness may exceed that of end-bearing pile. (2) For defected piles, the volumetric change has the positive relation with the variation of dynamic stiffness. (3) Dynamic stiffness is very sensitive to necking defection, which can be used to estimate the pile integrality and bearing capacity to some extent.