Abstract: Aiming at the problem that the swelling force of the red-bed mudstone is smaller than the theoretical value according to the code, this paper develops a triaxial swelling force device for the swelling rock. By placing the sample first and then restraining it, the deformation gap between the rock and the container’s wall is overcome. Through the experimental research of the triaxial expansion force of expansive soil and red-bed mudstone, the rationality and effectiveness of the triaxial expansion force device are verified. According to the experimental test results of the typical red-bed mudstone samples in Sichuan, the changes of the swelling force of the muddy swelling rock is preliminarily summarized. The research results show that the triaxial expansion force device and its test method can overcome the limitations of the existing method of consolidation instrument test. It is simple to operate, is of good practicability, and can be applied to the triaxial expansion force test of expansive soil and expansive rock. The experimental results of comparison of triaxial expansion force and uniaxial expansion force show that the triaxial expansion force of the red-bed mudstone is mostly in the magnitude of MPa, and the expansion force tested by the consolidation instrument is mostly in the magnitude of kPa. Through the analysis of the test results and the observation of the sample after expansion, it is found that the expansion mechanism of the red-bed mudstone is different from that of the expansive soil. The red-bed mudstone is mainly the surface layer of fresh rock that absorbs water and swells and disintegrates. After the surface layer is denuded, the fresh surface absorbs water again to swell and disintegrate. This is a cyclic expansion process layer by layer.