Rock wettability is the key factor affecting the microscopic distribution of fluids in rock pores and the interaction between fluid and rock. However, the shale from the Longmaxi Formation (LF) in Sichuan Basin is rich in organic minerals, which make the quantitative test of mixed wettability of shale very difficult. In this study, firstly, the micro-pore structure of shale was quantitatively characterized by scanning electron microscope (SEM). Secondly, a new experimental apparatus for shale water spontaneous imbibition under the conditions of formation temperature and confining pressure was designed. Finally, the wettability index was proposed to quantitatively evaluate the water wettability, oil wettability and mixed wettability of shale. The experimental results shown that the organic-rich shale has complex mixed wettability, which was both water-wet and oil-wet. The contact angle of shale samples tested with distilled water were 13.5° and 70.5°, and the contact angles tested with kerosene were 0° and 9.6° at room temperature. The higher temperature the stronger shale hydrophilicity will be, and the water wettability of shale increases with the imbibition time.Therefore, a large amount of fracturing fluid spontaneously permeates into the shale micro-nano pores in the process of hydraulic fracturing, which is an important reason for the low flowback efficiency of shale gas wells. The research results provide effective guidance for the optimization of flowback system.