Based on characteristics of multiple pores in the shale of Longmaxi formation (LF) in the south of Sichuan Basin, an analytical model of imbibition was established. Imbibition depths of different pore types were quantitatively calculated. Water imbibition experiments under conditions of formation temperature, confining pressure, and fluid pressure were carried out. Water imbibition law in shale was analyzed. It shows that according to imbibition saturation, the shale forcible imbibition can be divided into 3 periods. They are imbibition diffusion, imbibition transition and imbibition balance periods, respectrvely. Among them, the imbibition diffusion period is the main period for imbibition capacity rise. In the early period of shut-in, the imbibition capacity of shale increases significantly under the action of fluid pressure, providing a large amount of imbibition fluid for the spontaneous imbibition later. The reservoir confining pressure has prohibition on shale imbibition, but even under reservoir confining pressure, imbibition can improve fracturing effect of reservoir, resulting in the increase of porosity of 0.42-1.63 times and increase of permeability of 17.6-67.3 times. The imbibition depth of clay pore is much greater than that of brittle mineral pore and organic pore. Clay minerals are the main controlling factor of microfractures induced by shale hydration. An appropriate shut-in treatment enhances dramatically fracturing performance in shale gas reservoirs of LF in southern Sichuan Basin. The study provides scientific basis for the optimization of flowback regime of shale gas well.