Disclosed is a method and apparatus for manufacturing an optical fiber preform, using an outside vapor deposition (OVD) process, in which deposition and sintering processes can be continuously carried out in an OVD apparatus. The manufacturing apparatus includes a vertically-extending carriage, and a sintering unit installed at the upper end of the carriage, and adapted to sinter a clad deposited on a circular target rod. The sintering unit is a hydrogen/oxygen flame burner or a furnace using a heating source that doesn't generate H20 or hydroxyl groups (OH) during a heating operation thereof The manufacturing method includes a deposition step of depositing a clad on the circular target rod while reciprocating a deposition burner, and a sintering step of sintering the clad while reciprocating the sintering unit in a state in which the circular target rod deposited with the clad extends through the sintering unit.