The invention provides a two-stage pressurizing device based on uniaxial loading for an in-situ neutron diffraction experiment. The two-stage pressurizing device comprises two first-stage anvil cells with opposite anvil faces and a seal located between the anvil faces of the two first-stage anvil cells. A portion in the seal on the opposite anvil faces of the two-stage anvil cells is punched as a first-stage pressure cavity; a second pressurizing ring is arranged in the first-stage pressure cavity; the outer wall of the second pressurizing ring coincides with the inner wall of the first-stage pressure cavity; the outer diameter of the second pressurizing ring is smaller than or equal to the inner diameter of the first-stage pressure cavity; second-stage pressurizing sheets, the diameters of which are equal to or slightly smaller than the inner diameter of the second-stage pressurizing ring, are separately arranged on upper and lower openings of the second-stage pressurizing ring; a space encircled by the second-stage pressurizing ring and the second-stage pressurizing sheets become a second-stage sample cavity for holding a sample; after sampling in an experiment, the two second-stage pressurizing sheets are partially or fully embedded into the second-stage pressurizing ring, so that the problem that the sample cavity punched in the seal is expanded as a result of transverse extrusion of the sample is solved, and the problem that the sample is thinned as the pressure is increased in the in-situ neutron diffraction experiment is also solved.