The invention discloses a testing device and method for simulating deep rock mass impact failure. The testing device comprises an impact mechanism and a confining pressure loading mechanism, wherein the impact mechanism comprises a gas gun tube, a sabot and a flyer, the confining pressure loading mechanism comprises a confining pressure cylinder body, a through hole is formed in the middle part of a cover of the confining pressure cylinder body, two confining pressure testing sensors are arranged on the inner wall of each semicircular cylinder body of the confining pressure cylinder body, a blocking piece vertical to the axial line of the confining pressure cylinder body is arranged in the confining pressure cylinder body, a sample to be tested is clamped between the blocking piece and the cylinder bottom, a hoop is sleeved at the outer wall of the cylinder body, a conducting wire hole and a grouting hole for injecting an expansive cement mixture are formed in the cylinder bottom, and the expansive cement mixture is composed of expansive cement, water and iron sand or expansive cement, water and rubber powder. The method comprises the steps of assembling the testing device, debugging the assembled device, and impacting the blocking piece through the flyer. According to the invention, the proportions of the raw materials of the expansive cement mixture are adjusted to simulate the environment of deep rock mass, and the testing device has the advantages of high safety, good sealing property, stable confining pressure load, convenience and quickness for operating, and the like.