The knowledge of distribution of pore space inside grain bulks is essential for determining the airflow resistance of grains. In this study, the internal pore structure and the 3D-distribution of air paths inside grain bulks were studied using X-ray computed tomography images. Image analysis methods were applied to the binary 3D X-ray CT images on the spatial distribution of voids to generate the connected, individualized pore objects of different size and shapes. Morphometric parameters, such as 3D air path volume distribution, structure separation factor, Euler number, fragmentation index, and structure model index were calculated based on hexahedral marching cubes volume model and marching cubes 3D surface construction algorithm. The quantified numerical measures of spatial integrity of air path networks were analyzed and compared with the airflow resistance of grain bulks. The results showed that the connectivity of airspace and the nonuniform distribution of air-path network inside grain bulks were responsible for the difference in airflow resistance between horizontal and vertical directions to the airflow of grain bulks.