The invention relates to a method for preparing a loose optical fiber preform by use of multiple blowlamps. According to the method, mixed gases containing SiCl4 are introduced into the blowlamps according to a predetermined ratio and a predetermined flow to have oxidation and hydrolysis reactions, and SiO2 generated in the reactions are deposited on the surface of a core rod to form the loose optical fiber preform having a certain weight; the method is characterized in that the number of the blowlamps is 5-15, the blowlamps move back and forth in a horizontal direction relatively to the core rod from a starting point, after multiple reciprocating motion of the blowlamps, the blowlamps move back and forth oppositely to the core rod in a direction facing the starting point and go back to the starting point, and therefore, a reciprocating motion cycle is completed; and next, the reciprocating motion cycle of the blowlamps is repeated until the weight of the deposited loose optical fiber preform reaches the required weight. The method has the advantages of increasing the deposition rate and keeping the outer diameter deviation of the prepared loose optical fiber preform small.