[Purposes] The paper aims to figure out the development regulation and anisotropy of mechanical properties of 3D printed slag?fly ash?based geopolymer concrete with age. [Methods] The cubic compressive strength, axial compressive strength and flexural strength of printed geopolymer concrete at different curing ages and along x, y and z loading directions were tested in this study. X?ray CT scanner was used to analyze the microstructure and clarify the influence mechanism on the mechanical properties of printed concrete. [Findings] The mechanical strengths of printed concretes at 3 and 7 d were more 30% and 60% than that at 28 d, respectively. The printed concrete had an early strength and met the requirement of concrete printing construction. The differences in cubic compressive strength, axial compressive strength and flexural strength of the specimens cured for 28 d in each loading direction were in the range of 3.20%-10.40%, 1.53%-7.05% and 5.10%-24.00%, respectively. [Conclusions] The printed concrete exhibited certain anisotropy in mechanical performance, mainly due to the number and spatial distribution of defects in the material during the stacking process of the printed concrete.