The invention discloses a device for testing the performance of a cohesive soil sample. The device comprises a sample box, an upper permeable stone, a lower permeable stone, upper filter paper, lower filter paper, a micro differential pressure sensor, a data acquisition instrument, a precision injection pump, a first liquid storage tank, a second liquid storage tank, a third liquid storage tank and a fourth liquid storage tank, wherein an upper left way pipe, an upper middle way pipe and an upper right way pipe are provide in a top cover, a lower left way pipe, a lower middle way pipe and a lower right way pipe are provide in a pedestal, the micro differential pressure sensor is connected with the data acquisition instrument, and four injectors are arranged in the precision injection pump. The testing device can carry out determination of the coefficient of membrane efficiency, the coefficient of effective diffusion and the performance of retardation factors on the cohesive soil sample. Meanwhile, the invention also provides a testing method used by the device for testing the performance of the cohesive soil sample, and the method is simple and practicable and can realize combined determination of the coefficient of membrane efficiency, the coefficient of effective diffusion and the performance of retardation factors of the cohesive soil sample instead of respective determination, thereby improving determination efficiency.