The invention discloses a device and a method for an X ray transmittable test for simulating in situ crushing of rocks. The device comprises a main loading mechanism, a pressure head driving mechanism, a rock-crushing working surface slurry pressure simulation mechanism and a sample crushing test mechanism; the main loading mechanism is composed of a base, an axial pressing piston, a PEEK pressure-bearing drum, a blind flange and a cushion column; a pressure head loading rod is penetrated through the cushion column and the blind flange and is used for driving a pressure head to act on an end face of a rock sample and to insert into the sample for crushing. Axial pressure and confining pressure are loaded to a test design value; in situ three-directional stress state of rock in rock mass engineering is simulated; an unloading working surface of the rock mass engineering is simulated by the end face of the sample in an area in the middle of the cushion column; liquid can be injected to achamber on the upper part of the area, so as to simulate the slurry pressure of the working surface of the rock mass engineering. The pressure-bearing drum is prepared from X ray transmittable PEEK engineering plastics. Combined with an opening type CT scanning instrument, the device is capable of monitoring the microcosmic crack initiation, extension and converging process in the rock in real time, for analyzing the development mechanism of rock crushing cracks.