The invention discloses a synchronous powder feeding space laser machining and three-dimensional forming method and device. A three-dimensional solid is divided into a plurality of forming units according to the form simplification and nozzle fusion covering scanning accessibility principle, each forming unit is divided into a plurality of laminates, a mode for single-beam gas-carried powder feeding in a hollow annular laser is adopted, a mechanical arm is controlled to drive an in-laser powder feeding nozzle to move for scanning in filling areas and boundary areas of the laminates according to a preset trajectory, and forming fusion covering and stacking forming of the laminates are sequentially completed, so that forming fusion covering and stacking forming of the whole unit are completed. The device comprises the in-laser powder feeding nozzle, a laser generator, the mechanical arm, a control module, a transmission optical fiber, a gas-carried powder feeder and a gas source. The control module is connected with the mechanical arm, the laser generator and the gas-carried powder feeder. The in-laser powder feeding nozzle is fixed to the front end of the mechanical arm and can make spatial motion along with the mechanical arm. Through the method and device, fusion covering and three-dimensional stacking forming on any surface of the space can be achieved, and supportless three-dimensional forming of parts with complex structures such as a hanging structure and a cavity can be achieved.