Abstract: Shale gas development is relied on long horizontal drilling. But shale is typical formation that wellbore instability often occurs, restricting long horizontal drilling in shale reservoir. Hydration damage is a major trigger of borehole collapse in shale formation. During drilling process, spontaneous imbibition is one of main methods of water phase in drilling fluid entering into shale. After the invasion of water phase into shale, hydration between water and clay happens, forming damage effect. Currently, there are plenty of works about shale hydration mechanism, but research of connection between spontaneous imbibition and hydration is rare. The understanding of shale hydration evolutionary law during spontaneous imbibition is not deep enough, restricting the improvement of horizontal drilling speed and drilling quality in shale reservoir. Therefore, in combination with spontaneous imbibition test and rock mechanical test, based on damage mechanical theory, characteristics of shale spontaneous imbibition and hydration damage evolutionary law during spontaneous imbibition have been clarified. The constitutive model of shale hydration damage during spontaneous imbibition has been built. The analysis of shale energy response law has been conducted. Results indicate that shale has strong spontaneous imbibition in earlier stage, and the imbibition gradually becomes stable at later stage. Damage degree is consistent with degree of spontaneous imbibition. During spontaneous imbibition, hydration crack starts to propagate, decreasing mechanical strength and leading to the change from brittleness to plasticity. With increasing imbibition time and hydration degree, total energy, elastic energy and dissipated energy all show decline, indicating the decreasing mechanical stability. Outcomes improve the understanding of shale mechanical behavior, offering theoretical support for high efficiency and safety of long horizontal drilling technique in shale gas reservoir.