A light-curing 3D printer includes a main-tank, a printing platform, a lighting unit, a liquid material contained in the main-tank, an isolation fluid contained in the main-tank and floating upon the liquid material, a membrane arranged upon the isolation fluid, and an auxiliary mechanism. The 3D printer controls the lighting unit to emit light toward the liquid material according to slicing data of one cured-layer of a 3D model for forming a 3D object. The 3D printer then controls the printing platform to lower and activates the auxiliary mechanism to keep varying the status of the isolation fluid. Next, the 3D printer determines whether the 3D model is completed, and controls the lighting unit to emit light according to slicing data of a next cured-layer if the 3D model is not yet completed.