The invention discloses a tunnel seepage instability model test device and test method with the ground pressure and the water pressure controllable. The device comprises a medium filling chamber, a loading piston box, a loading system and a water injection system; a reserved hole is formed in the lower portion of the side wall of the medium filling chamber; the loading piston box is arranged at the ceiling of the medium filling chamber, a water inlet is formed in the top of the loading piston box, multiple water seepage holes are formed in the bottom of the loading piston box, the loading system is arranged at the top of the loading piston box, and the water inlet is connected with a pipeline of the water injection system. Through low-capacity filling and repeatable operability of the test device, possibility is provided for researching the seepage failure forms of a tunnel under different ground-pressure and water-pressure conditions through a great number of tests, the ground stress can be loaded to a small-size tunnel seepage instability model in a stable and long-acting mode under a pressure water environment, consistency between the model test conducting condition and the simulation engineering condition is guaranteed, the test device can be accurately applied to simulation of complicated geological conditions such as a tunnel and a mine, and powerful guarantee is provided for research of seepage instability mechanisms.