Shale rocks are multi-component materials with complex interactions among different components. As a result, shale wettability evaluation becomes extremely difficult and knowing key factors that control shale wettability is particularly important for shale reservoir development. In this paper, wettability of both shale surface and pore of Longmaxi Formation in the southern Sichuan Basin were comprehensively evaluated by low field nuclear magnetic resonance (NMR) together with experiment; a new concept, mixed wettability index, was introduced to quantitatively evaluate the mixed wettability and clarify the key factors affecting shale wettability. The test results of wettability of different shale samples by the new method show that the surface of the Longmaxi Formation shale exhibits complex non-uniform mixed wettability, both oil-wet and water-wet, while the shale surface tends to be more oil-wet, the contact angles range from 15.9 ° to 63.7 °, with an average of 41.3 °, when tested with distilled water, and the shale is more water-wet under formation temperature. The shale pore also exhibit complex mixed wettability. The clay mineral content is the dominant factor affecting water wettability of shale, but when reaching beyond a certain value, it inhibits shale water wettability to some extent. The organic carbon (TOC) is the dominant affecting factor of oil wettability and positively correlated with oil wettability of shale. The research results are helpful for making clear the micro mechanism of the action between fracturing fluid and shale reservoirs and designing fracturing scheme.