The invention provides a method and device for quantitatively characterizing wettability of rocks in a shale reservoir. The method comprises the following steps: obtaining shale in the shale reservoir, and preparing a shale sample; performing a spontaneous infiltration experiment on the shale sample to obtain a self-priming slope; calculating a wettability index WI by using WI=(P1-T1)-(P2-T2); performing quantitative characterization evaluation on the wettability performance of the shale sample: the wettability index is within the range of -0.5 to 0; if the wettablity index is closer to -0.5,the water dampness behavior is stronger, otherwise, the shale sample is closer to blended water dampness; when the wettability index is 0, the shale reservoir is in blended dampness; the wettability index is within the range of 0 to 0.5; if the wettability index is closer to 0.5, the oil dampness behavior is stronger, otherwise, the shale sample is closer to blended oil dampness. According to themethod, by introduction of a new wettability index, the wettability of the shale reservoir can be accurately and quantitatively characterized; the method has important significance for designing an effective yield increase technology and estimating the ultimate recovery factor.