The invention discloses a rock three-way pressure stress sensor based on optical fiber grating sensing. The rock three-way pressure stress sensor comprises a cubic box body, wherein the box body is provided with six surfaces; three of the six surfaces which share a vertex are stress surfaces; each of the other three surfaces which share the vertex is provided with a recessed disc and serves as a recessed disc surface; fastening screw holes which have the same size and are uniformly distributed are formed at the periphery of each recessed disc; an optical fiber channel is formed in the center of each recessed disc; a sensing film is connected to each recessed disc; a small circular thin plate in the middle of each sensing film is an effective pressure bearing surface, and an optical fiber grating is adhered to an inner surface of the small circular thin plate; a sensing film supporting end is arranged at the periphery of the effective pressure bearing surface; screw holes which have the same size and are uniformly distributed are formed at the periphery of the supporting end; a box body penetration hole is formed at each vertex of the stress surfaces and the recessed disc surfaces;and the optical fiber channels are connected with two box body penetration holes. The rock three-way pressure stress sensor is intrinsically safe, free from electromagnetic interference, high in long-term stability, large in range, high in accuracy, high in sealing performance, pressure-resistant, corrosion-resistant and the like.