The invention discloses a multiscale three-dimensional digital core constructing method based on CT images with different resolutions. The method comprises the following steps: 1) three-dimensional digital cores with high resolution and low resolution are obtained by performing different-resolution scanning on a core; 2) a three-dimensional pore space network of the core is extracted with a maximal ball algorithm on the basis of the three-dimensional digital cores, and a coarse-scale pore network model and a fine-scale pore network model are obtained respectively; 3) the coarse-scale pore network model extracted with the maximal ball algorithm is transplanted directly to serve as a coarse-scale model of a novel model; an equivalent fine-scale pore network is reconstructed randomly through combination with geometric topological structure information of a fine-scale network; 4) the coarse-scale pore network model and the equivalent fine-scale pore network are stacked through trans-scale coupling of the pore networks. The digital cores constructed with the method are in higher accordance with characteristics of the actual core, and the multiscale three-dimensional digital core is constructed by fusing the CT images with different resolutions.