A pressure-preserving conventional triaxial compression loading apparatus of the present invention includes a pressure vessel, an upper piston rod, a lower piston rod, and an annular oil bag assembly. Hollow chambers of the pressure vessel in vertical communication sequentially include an upper chamber, an upper sealed chamber, a confining pressure chamber, a lower sealed chamber, and a lower chamber from top to bottom. The annular oil bag assembly is placed in the confining pressure chamber. When an annular inner chamber of an annular oil bag is filled with medium, an outer wall of the annular oil bag and an inner wall of the confining pressure chamber are attached together. A fidelity specimen is placed in a specimen chamber defined by a lower end surface of the upper piston rod, an upper end surface of the lower piston rod, and an inner wall of the annular oil bag. A variety of measuring sensors are disposed in the annular inner chamber of the annular oil bag. The pressure-preserving conventional triaxial compression loading apparatus of the present invention may accommodate a fidelity specimen, and use the annular oil bag assembly and the upper and lower piston rods to perform a pressure-preserving conventional triaxial loading test on the fidelity specimen, so that test data is more accurate and reliable, to help to study the mechanical behavior of in-situ rock and measure their properties more faithfully.