Abstract: Vertical tests are very important for superconducting cavity after its post-processing. The aim is to obtain the quality factor versus accelerating gradient curve to evaluate a cavity’s performance. Because of its narrow bandwidth, the superconducting cavity should work stably in the resonant state during the vertical tests. The digital self-excited loop system for the vertical test stand of Peking University is introduced in this paper. The methods of avoiding crosstalk during multi-cell superconducting cavity test were brought out. The influence of deviation from quadruple frequency sampling on amplitude and phase was analyzed. The system is stable and reliable, can effectively distinguish \textπ mode from 8\rm\textπ/9 mode of the 1.3 GHz 9-cell superconducting cavity, and solve the problem of mode crosstalk in multi-cell superconducting cavity test.