The invention discloses a device for measuring axial displacement of a solid rocket engine grain in the launching process. The device comprises a body, fixed blocks and a reference block. The body is a test engine. The front end and the rear end of the surface under a shell of the test engine are each provided with a gap, the parts of the internal grain are exposed out of the gaps to be used as adhesive connection areas of the front and rear fixed blocks, each fixed block is of a U-shaped structure, the bottom face of the first end face of each fixed block is a circular arc coinciding with the outer diameter of the grain in the circumferential direction, the fixed blocks are arranged in the front and rear gap areas in the surface of the shell and used for being adhesively fixed to the grain, and the second end face of each fixed block provides an installation port for the contact end of a displacement meter through a through hole. The bottom face of the reference block is a circular arc coinciding with the outer diameter of the shell in the circumferential direction, the reference block is arranged on the surface of the shell, used for being adhesively fixed to the shell and located between the front fixed block and the rear fixed block and provides installation ports for the body of the displacement meter through two penetrating type T-shaped through holes in the reference block. By means of the device, the environmental suitability and working reliability of an engine under guided missile vertical cold launching conditions are improved.