The present invention relates to a tri-axial mechanical test apparatus and method for simulating the process of freezing the high-pressure water into ice. The tri-axial mechanical test apparatus comprises a main body loading system, a freezing system, and a sample test system; in the main body loading system, a flange, an axial pressure piston and a pressure-bearing shell are constituted to a loading main body, and the axial pressure and confining pressure are controlled directly by a tri-axial servo tester and indirectly by an oil-water separator respectively; in the freezing system, a circumferential freezing liquid circulation channel and a base freezing liquid circulation channel are connected to an external cold source for cooling and freezing, and a dissoluble shell is provided on the periphery of the sample to ensure the shape of ice; in the sample test system, a serial of optical fiber sensors in the sample is connected to an optical fiber data collector, to measure temperature and strain. The present invention cooperates with a tri-axial servo tester, an oil-water separator, an external cold source and an optical fiber data collector, so that the water can be frozen into ice under pressure under the confinement of a dissoluble shell that is dissolved after an ice sample is formed, and then the tri-axial mechanical test can be carried out directly in the original stress state after water is frozen into ice under pressure.