The invention discloses a test device and method for determining dislocation interface seepage failure hydraulic gradient through unidirectional flow in-situ test. The disadvantage in the existing technical scheme of determining the dislocation interface seepage failure hydraulic gradient is overcome, and a method for determining the dislocation interface seepage failure hydraulic gradient according to an unstable slow formula analytic solution in the dislocation interface under a ladder boundary water level condition based on the unidirectional flow in-situ test device. The device comprises awater inlet bin, a water outlet bin, a piezometric tube, a pressure instrument and a water injection pressurizing device. The method comprises the following steps: selecting an appropriate test pointin field adit to form an in-situ test sample; developing the field test, recording the flow data and the failure omen time; acquiring dislocation interface seepage parameter based on the test data; and computing the dislocation interface seepage failure hydraulic gradient.