The invention provides an existing 
engineering pile bottom depth determination method based on a parallel seismic inflexion-
point method in the field of 
civil engineering pile foundation construction quality inspection. Detection is carried out in an exploratory hole after the exploratory hole is drilled beside a 
pile, a time-depth 
signal diagram is generated according to different depth signals in the 
depth direction, an inflection point is determined in the time-depth 
signal diagram, and the pile bottom depth is obtained after the depth of the inflection point is modified through a inflection-point depth modifier formula. Through the method, the influence on 
wave form from an upper structure is avoided, the inspection is convenient, and the determination of position of the inflection point is simple and visual. Compared with the conventional parallel seismic method, by which the pile bottom depth is determined based on intersection of two lines, the parallel seismic inflexion-
point method has a lower requirement on the depth of the 
drill hole, just 2-3 m deeper than a pile to be inspected, thereby reducing the requirement on the depth of the exploratory hole, saving cost, having better maneuverability for technical personnel without 
engineering experience, and better solving the current problem that the pile depth of an 
engineering pile is hard to determine.