Abstract: In order to deeply explore the damage morphology, crack extension and interlayer relationship of 3D printed steel slag cementitious materials, taking the change of fractal dimension of materials before and after uniaxial compression as the focus of the study, CT image technology was used to obtain 2D images loaded to 90% of the predicted damage load, and the microscopic damage cracks were extracted with the help of image processing software. Based on the theory of fractal geometry, the fractal dimension was used to reveal the extension law of cracks in the interlayer, and the relationship between fractal dimension and crack characteristics was establish. Based on the fractal geometry theory, the fractal dimension was used to reveal the crack expansion law between layers and establish the relationship between the fractal dimension and the crack characteristics. The results show that: 3D printed materials show more brittle and lower strength eigenvalues than cast materials; the changes of porosity, pore diameter, crack fraction and crack width of 3D printed steel slag cementitious materials are affected by the interlayer variations; Fractal dimension quantitatively describes the expansion of internal cracks, and the linear correlation between fractal dimension and crack fraction and crack area is relatively high, while the correlation between crack length (width) is relatively weak. This study provides a theoretical and experimental basis for the improvement and study of the interlayer properties of 3D printed cementitious materials.