The invention relates to a device and method for preparing an optical fiber preform by VAD (vapor axial deposition). The device comprises a vertical machine frame, wherein the vertical machine frame is provided with a lifting chuck; the lifting chuck is connected with a cantilever support; the front end of the cantilever support is provided with a rotary hanger rod; the lower part of the rotary hanger rod is correspondingly provided with a reaction chamber for deposition; and a core blowtorch and a coating blowtorch are respectively arranged in the reaction chamber. The device is characterized in that a laser range finder is arranged outside the reaction chamber; the range finding point of the laser range finder is corresponding to the preform core deposition formation position above the core blowtorch; and the laser range finder is used for detecting the diameter of the preform core. The device has the advantages of simple structure, high measurement accuracy, high feedback speed and high reliability, and is easy to operate and use. The device can effectively control the core diameter of the deposited preform, so that the produced optical fiber preforms have uniform core diameter, thereby enhancing the processing quality of the preforms.