A 3D printing apparatus and a method thereof are provided. The apparatus includes base, forming platform, nozzle module, curing module, first sensor and second sensors and control module. The forming platform is movably disposed on the base along an axial direction. The nozzle module and the curing module are disposed on the base and located above the forming platform. The first and second sensors are movably disposed on the base and located at two opposite sides of the nozzle module and the curing module along the axial direction. A predetermined range of the 3D object on the forming platform has first and second endpoints along the axial direction, and the first and second sensors respectively correspond to the first and second endpoints. The control module determines whether the nozzle module impacts the 3D object according to a sensing time difference of the first and second sensors.