The invention relates to a preparing method for a low-porosity 3D printing product. A composite material which is extruded from a nozzle a of a 3D printing machine A and provided with a skin core structure is deposited on a bottom plate, and the low-porosity 3D printing product is obtained; and a core layer of the composite material with the skin core structure is solidified in advance of a skin layer in the 3D printing process, and the porosity of the low-porosity 3D printing product ranges from 1% to 10%, wherein the content of the skin layer of the composite material with the skin core structure ranges from 10 wt% to 50 wt%, the melting point of the skin layer ranges from 50 DEG C to 120 DEG C, the melting point of the core layer ranges from 100 DEG C to 400 DEG C, the melting point ofthe skin layer is lower than that of the core layer by 15 DEG C to 330 DEG C, the temperature of the nozzle a is higher than the melting point of the core layer by 10 DEG C to 50 DEG C in the 3D printing process, and the temperature of the bottom plate is higher than the melting point of the skin layer by 10 DEG C to 30 DEG C. By means of the method, the skin layer of the product is firstly meltedand then permeates into pores of the product, the porosity of the 3D printing product is reduced, roughness of the product is lowered, and the tensile strength of the product is improved.