The invention relates to a device and a method for preparing optical fiber preform with a VAD (vapor axial deposition) method. The device comprises a reaction chamber and a vertical rack, wherein a vertical lifting seat is arranged on the vertical rack and connected with a rotating suspender, a gas inlet end is arranged on one side of the reaction chamber, a gas outlet end is arranged at the other end of the reaction chamber, a core blow lamp and a cladding blow lamp are arranged in the reaction chamber, an opening corresponding to the core blow lamp and the cladding blow lamp is formed in the upper part of the reaction chamber and connected with an upper chamber, and the rotating suspender extends into the upper chamber. The device is characterized in that the gas outlet end is connected with a cladding gas outlet pipeline and a core gas outlet pipeline respectively, the cladding gas outlet pipeline is located above the gas outlet end, and the core gas outlet pipeline is located below the gas outlet end. The stability of core flame and a core deposition zone is guaranteed through control of a pressure feedback system in a deposition process. The stability of the flame in the core deposition zone can be effectively guaranteed, deposition of the preform core is stable and controllable, so that basic parameters such as the diameter of the preform core are uniformly distributed along the preform length, finally, the stability of the core diameter is guaranteed, and the manufacturing quality of the preform is improved.