Abstract: When utilizing the reflected wave method for integrity testing of piles, signals are usually biased due to the three-dimensional (3D) wave motion of the piles. In order to reduce the signal bias, both the 3D interference sources and the optimal sampling positions are studied. By means of the mode superposition, it is observed that the interference sources are mainly from a specific kind of vibration modes, in which the top plane of a pile will deform non-uniformly in the axial direction. There exist stationary points on the top plane where the interference signal is the minimum. They are called 3D interference minimum points, the optimal positions for signal sampling. Through parameter analysis, a mathematical expression is derived for determination of the 3D interference minimum points. This expression reflects the effect of pile materials, and may be regarded as a modification of the existing code method. Based on the fact that the displacements at both sides of an interference minimum point are adverse, a double-speed average method is proposed, by measuring and averaging the signals of two corresponding points at both sides. Using this method, the sampling location can be chosen in a wide range, while the signal quality is much higher than that of the 3D interference minimum point sampling method. The accuracy of the proposed method is further checked through both the in-situ tests and the numerical simulations.