An unsupported 3D printing system is characterized in that photosensitive resin can be subjected to reversible conversion between liquid and a paste; a projection device adopts UV projection as a light path programming mechanism, the projection mode of the projection device is face forming, and the projection device can move transversely or longitudinally under driving of a projection driving device; a printing platform is arranged in a material groove; and a temperature control device is arranged outside the material groove, a hot medium or a cold medium circulates in the temperature controldevice and the temperature control device can heat or cool the material groove. The photosensitive resin capable of being subjected to reversible conversion between the liquid and the paste is adopted, along with superposition of printing layers, the photosensitive resin in the paste state can also be accumulated layer by layer to form a support to support a printed solid material, thus the process of manual additional arrangement of supporting columns in the modeling process and the process of manual supporting removing after printing is completed are omitted, the printing efficiency is improved, and the labor cost is reduced.