Abstract: Parallel Seismic (PS) test has been widely used in pile length and defect detection.Existing pile foundations work in the loaded environment but the research on the applicability of pile length measurement by Parallel Seismic Test is still rare.The ABAQUS finite element model was carried out to test the length of existing pile foundation by Parallel Seismic (PS) test.Combined with theoretical analysis，the parameters of the existing building load，pile length，pile-to-borehole depth and pile-to-borehole distance by Parallel Seismic Test were studied.Then，the correction method of pile length detection error under different conditions were determined.The results show that when the pile foundation is in the linear elastic working stage，the influence of the upper load on the determination of pile length by the PS test method is negligible，indicating that PS test has good applicability to the test of the pile length of existing buildings.However，in the application，attention should be paid to the influence of the pile-to-borehole depth and pile-to-borehole distance.The "correction method" has a higher precision when the hole depth is sufficient，and the error is within 10%.，and the "intersection point translation method"has less error when the pile hole distance is small，no more than 11%.The error of the "inflection point method" is less affected by the hole depth，and the precision is higher.The maximum error is 7% when the hole depth is insufficient，and the "intersection method" error is usually larger，exceeding 10%.The research results can provide a theoretical reference for the application of the Parallel Seismic (PS)test in the test of pile foundations with existing buildings.