3D printing mortar generally has the problems of weak interlayer interface bonding and low flexural strength. The expansion cement slurry coated on the surface of mortar can produce surface compressive stress and then improve its flexural strength. As an interface agent, coating between layers can simultaneously enhance the interfacial bond strength. In this paper, the coating composed of sulphoaluminate cement and expansive agent was coated on the surface and interlayer of the 3D printing mortar, and the effect of the coating on the mechanical properties of the 3D printing mortar specimens without fiber and with 0.5% (volume fraction) basalt fiber was studied. The experimental results show that the interlayer coating improves the interlayer interfacial bond strength of fiber-free and fiber-doped rapid setting 3D printing mortar by 21.4% and 12.2%, respectively. At the same time, it also improves the compressive strength. The effect of coating only surface on the flexural strength of the 3D printing mortar is similar to that of coating only between layers. At the same time, surface coating and interlayer coating have the most obvious improvement effect on the flexural strength of the 3D printing mortar. Compared with the fiber-free and fiber-doped non-coating specimens, the highest improvement rates are 44.2% and 23.2%, respectively. The effect of coating on the interlayer interfacial bond strength and flexural strength of fiber-free 3D printing specimens is better than that of fiber-doped specimens.