To figure out the influence mechanism of extruded strip shape and dimension on the performance of 3D printed concrete, the strip shapes of triangle, square and circle at the same extrusion flow rate, the extrusion diameter, and the printing arrangement using a same shape were typically selected to compare with the mold forming specimens on the fracture surface, mechanical performance and the spatial pores distribution. The characteristics of layer/strip defects in printed specimens at different shapes and dimensions of extruded strip were analyzed in detail and the spatial distribution of pores was discussed, the relationship between porosity and mechanical properties was clarified. The experimental results indicate that the compactness of printed concrete is mainly determined by the intersectional defects between layers and strips. At the same extrusion flow rate, the specimen stacked with triangle extrusion strip achieve the lowest porosity and the best mechanical properties. With the increasing dimension of extruded strip, the defects in printed concrete is decreased, resulting in an increase of mechanical property. At the same shape and size, the strengths of printed concrete can be improved by 13%47% due to the dislocation arrangement of upper and lower strips.