Abstract: The strength and deformation characteristics of mudstone are important reference indexes in the engineering design and construction management. In order to analyse the effect of water-rock interaction and unloading on the strength properties and deformation characteristics of expansive mudstone, the sample of typical expansive mudstone in Nanning basin is chosen for the loading and unloading shear test and unloaded expansion ratio test. The intensity change rule of unloading mudstone is analyzed. And the influence of water-rock interaction on the deformation characteristics is discussed. The deformation and the curve of unloaded expansion ratio test are obtained. The experimental results show that sampling unloading has a great influence on the strength and compressibility, as well as the expansion deformation. Due to the unloading, the strength of mudstone decreases and the compression ratio of mudstone increases. The internal friction angle under unloading condition is smaller than that under the loading condition, while the value of cohesion increases. The unloading ratio and the loss strength rate are used to analyze the variation law of the strength of unloading mudstone. With the increase of the unloading ratio, the loss strength rate of unloading mudstone also increases. When the unloading ratio is between 0.7 and 0.8, the loss rate of mudstone strength is the largest. Compared with the unloading effect, the water-rock interaction has a greater influence on the deformation characteristics of unloading mudstone, and its structural damage to mudstone is up to 30% ~ 65% during the sampling progress. So, the influence of sampling process on the strength and deformation characteristics of mudstone can't be ignored.