In order to evaluate the applicability of the parallel seismic method in the detection of the length and integrity of existing pile foundation, the paper conducts dynamic analysis of the free pile foundation and the pile foundation with cap by numerical analysis method,and studies the influence of pile defects,pile-hole spacing and pile cap structure on the test results of the parallel seismic method. The results show that when the parallel seismic method is used to detect the length of existing pile foundation,the pile-hole test spacing should be less than 1 m;for the case of local defects in the pile body, when the test hole is close to the defect side, the slope of the deep straight line at the first wave changes obviously;for the existing pile foundation connected with the pile cap,the smaller the test distance between the pile and the test hole, the more obvious the abnormal characteristics of the waveform of broken pile and mud inclusion defects in the shallow part of the pile,but the parallel seismic method is difficult to identify whether there are defects at the pile-cap joint.