To explore the broken expansion and water storage evolution characteristics and internal mechanism of typical rock masses in goafs during mining, a synchronous test method of broken expansion coefficient and water storage coefficient based on a self-developed rock triaxial fluid-solid coupling test system was developed. A triaxial loading test was conducted on rock samples of coarse sandstone, siltstone, and mudstone to determine the broken expansion and water storage coefficients in complete stress-strain process, and the spatial distribution of internal fractures in the rock was synchronously obtained via nuclear magnetic resonance imaging. The results show that the broken expansion and water storage coefficients of the three types of rock samples gradually increase and tend to stabilize with the loading process, and the corresponding stress-strain curve shows a phased gradient change in four stages. Among them, the water storage coefficient exhibits a good Weibull distribution with strain variation. The initial porosity affects the expansion morphology of newly formed fractures and the water storage space of rock samples to some extent. As the initial porosity of rock samples increases, the water storage coefficient shows a linear increasing relationship before 80% prepeak, and then an exponential increasing relationship. The theoretical value of the water storage coefficient calculated based on the expansion coefficient is consistent with the evolution law of the experimental measured value, but the difference in value can reach up to 0.7%, indicating a clear characteristic of first broken expansion followed by water storage. The broken expansion and water storage coefficient of rock samples are influenced by the coupling effect of stress loading and initial porosity. In the initial stage of loading, the initial porosity dominates, gradually transitioning to stress dominance, and is ultimately determined by both stress and the degree of penetration of rock pores and fractures.