An apparatus for preparing a building structure with 3D printing, comprising: a 3D printing device, having a storage chamber and a printing head connected to the storage chamber and being movable relative to a base frame; a reinforcing device, comprising a cutting component having at least one cutter head for cutting a long rebar or long rib into a short rebar or short rib, movably arranged relative to the base frame and having one driving mechanism for driving the short rebar or rib to perform an insertion movement in a direction intersected with a stacking direction of a cement-based slurry layer; the driving mechanism inserting the rebar or rib into an interlayer interface spanning at least two adjacent cement-based slurry layers printed by the 3D printing device, the rebar or rib is the long rebar or long rib before being cut by the cutting component, and the rebar or rib is the short rebar or short rib after being cut by the cutting component; wherein the cutting component further comprises: a sleeve fixedly disposed relative to the driving mechanism and through which the rebar or rib pass; the cutter head, driven by a first driver, slides along an axis perpendicular to the sleeve and abuts on an end surface of the sleeve for extending the rebar or rib; characterised in that the reinforcing device further comprises a bending mechanism for bending at least one end of the short rebar or short rib parallel to the stacking direction of the cement-based slurry layer, after cutting by the cutting component, towards the direction intersected with the stacking direction of the cement-based slurry layer, and the driving mechanism drives the short rebar or short rib bended to perform an insertion movement. a 3D printing device, having a storage chamber and a printing head connected to the storage chamber and being movable relative to a base frame; a reinforcing device, comprising a cutting component having at least one cutter head for cutting a long rebar or long rib into a short rebar or short rib, movably arranged relative to the base frame and having one driving mechanism for driving the short rebar or rib to perform an insertion movement in a direction intersected with a stacking direction of a cement-based slurry layer; the driving mechanism inserting the rebar or rib into an interlayer interface spanning at least two adjacent cement-based slurry layers printed by the 3D printing device, the rebar or rib is the long rebar or long rib before being cut by the cutting component, and the rebar or rib is the short rebar or short rib after being cut by the cutting component; wherein the cutting component further comprises: a sleeve fixedly disposed relative to the driving mechanism and through which the rebar or rib pass; the cutter head, driven by a first driver, slides along an axis perpendicular to the sleeve and abuts on an end surface of the sleeve for extending the rebar or rib; characterised in that the reinforcing device further comprises a bending mechanism for bending at least one end of the short rebar or short rib parallel to the stacking direction of the cement-based slurry layer, after cutting by the cutting component, towards the direction intersected with the stacking direction of the cement-based slurry layer, and the driving mechanism drives the short rebar or short rib bended to perform an insertion movement. a sleeve fixedly disposed relative to the driving mechanism and through which the rebar or rib pass; the cutter head, driven by a first driver, slides along an axis perpendicular to the sleeve and abuts on an end surface of the sleeve for extending the rebar or rib; characterised in that the reinforcing device further comprises a bending mechanism for bending at least one end of the short rebar or short rib parallel to the stacking direction of the cement-based slurry layer, after cutting by the cutting component, towards the direction intersected with the stacking direction of the cement-based slurry layer, and the driving mechanism drives the short rebar or short rib bended to perform an insertion movement.