The relative displacement of the pile head which occurs when the pile soil system is impacted by the free-fall hammer is the dynamic response of the dynamic rigidity of the pile soil system to the acting load. During the low strain dynamic test of the foundation pile, the adoption of the velocity sensor to the direct measurement of the relative displacement of the pile head is helpful to reducing the improper dependence upon the measured intrinsic frequency valuesof the pile soil system in calculating vertical bearing capacity of the single pile, and is hence conducive to raising the accuracy of the calculated vertical bearing capacity of the single pile.