Disclosed in the present invention is a method for preparing a Y-branched hydrophilic polymer carboxylic acid derivative, in particular a method for preparing a Y-branched polyethylene glycol carboxylic acid derivative having a high purity and high molecular weight. The preparation steps are simple, the product of the reaction is easy to be separated, the cost for separation is low, and the purity and yield of the product are high, facilitating the subsequent preparation of other derivatives and medicament conjugates based on the preparation of the carboxylic acid derivative, which is advantageous for industrial scale-up and commercial applications. The prepared Y-branched hydrophilic polymer carboxylic acid derivative (in particular the Y-branched polyethylene glycol carboxylic acid derivative having a high molecular weight) product has a high purity and high commercial application value, in particular in the use of the preparation of medicaments for preventing and/or treating diseases.