Abstract A low strain integrity test is adopted to assess the quality of cast-in situ reinforcement concrete piles with high slenderness ratios. The disadvantage of traditional NDT equipment and signal post-process is that it cannot detect and interpret any structural faults that exist in long bored piles with high slenderness ratios. Due to insufficient impact energy, testing signal decay, soil–pile interaction and insufficiently homogeneous pile concrete material, etc., only the lower frequency portion of the test signal was used in nondestructive evaluation. In order to overcome these shortcomings, the testing devices have to first be adjusted for acquiring a lower frequency signal. Secondly, numerical signal process techniques (i.e., smoothing and amplifying) should be performed to enhance the reflection signals from the pile tip. The experimental results indicate that the testing signal identification abilities can be improved by both proper device adjustments and some signal process skills. A bored pile of up to 58 m in length with a slenderness ratio of 38.6 can be identified, even with a pile cap. Finally, a new proposed numerical signal process method, i.e., joint time–frequency analysis, is applied to explore the time–frequency component of the testing result which shows better resolution than traditional methods.