In order to study the mechanical properties of mudstone under water softening, Nuclear magnetic resonance test and uniaxial compression tests on mudstone were carried out. The results shows that: (1) The damage weakening process of water to mudstone is as follows: in the early stage of immersion, the small scale micro crack (the first peak of T2 spectrum) inside the mudstone gradually expands, and the larger size micro crack (the peak of the T2 spectrum second) increases and expands remarkably. With the increase of the soaking time, the micro crack relative to the larger size (the peak of the T2 spectrum second), with the rapid expansion, convergence and even penetration of the smaller size microcracks (the first peak of the T2 spectrum), was soaked to a long enough time, and the crack penetrated into a new larger size crack. At this time, the T2 spectrum distribution map appeared first. Three peak. (2) The damage weakening effect of water on the mudstone is mainly manifested by the increase of the immersion time, the further increase of the primary micro cracks in the specimen and the initiation of the new cracks. The two people gradually converge and through, thus providing more channels for water and further strengthening the damage of the water to the mudstone. (3) Water damage to strength parameters of mudstones is very significant. With the increase of soaking time, the uniaxial compressive strength and elastic modulus of mudstone decrease exponentially with the increase of immersion time.