Fly ash based geopolymer mortar for 3D printing （refered to as geopolymer mortar） was prepared with fly ash， slag powder and quartz sand as main raw materials， anhydrous sodium silicate as alkali activator and magnesium aluminum silicate as special additive. The effects of magnesium aluminum silicate dosages and rest time on rheological properties of geopolymer mortar were studied. The results show that the rheological properties of geopolymer mortar is improved with the increase of the dosages of magnesium aluminum silicate additive and rest time. It is found that fitting the rheological properties of geopolymer mortar with the Bingham model and the Herschel-Bulkley model， R² of the two models is satisfied， but the Bingham model is better than that of the Herschel-Bulkley model from comprehensive consideration. The geopolymer mortar mixed with 1.0% magnesium aluminum silicate admixture and resting for about 10 min can obtain better printability and printing effect.