Fused deposition modeling 3D printing can quickly manufacture parts and models with complex cavities using the layer-by-layer accumulation method, but the parts are prone to surface defects and dimensional errors in this manufacturing method. In order to solve the problem of rib drawing on the bottom surface of the overhang part during the printing process of the part, the cylindricity of the overhang part of the printed part was measured by adding a dense layer in the pre-treatment and the method of chemical solution polishing in the post-treatment, and the cylindricity measured by the direct printing part was taken as the benchmark. The results show that the cylindricity of the parts that have only been pre-treated is increased by 13.6% compared with the parts that have not been treated. The cylindricity of the post-treated parts was increased by 18.7% compared to the untreated parts. The cylindricity accuracy of the overhang part of the part obtained by combining pre-treatment and post-treatment is increased by 64.3%. The study verified that the combination of pre-treatment and post-treatment can effectively solve the tendon-like wire drawing in the overhang.