The influence of polymer on the apparent viscosity of 3D printing building mortar under different shear rates was studied. The thixotropy, plastic viscosity and yield stress of 3D printing building mortar were also investigated. The results show that hydroxypropylmethylcellulose(HMC) has a great influence on the apparent viscosity, thixotropy, plastic viscosity and yield stress of 3D printing building mortar; HMC with starch ether(KHC) compound, redispersible latex powder(FX) with KHC compound, or HMC, KHC with FX compound have a good synergistic effect; and when HMC mixed with KHC and FX at a suitable dosage, it is most beneficial to achieve good 3D printing properties.