Mainstream person search models aim to jointly optimize person detection and re-identification (ReID) in a one-step manner. Despite notable progress, existing one-step person search models still face three major challenges in extracting discriminative features: 1) incomplete feature extraction and fusion hinder the effective utilization of multiscale information, 2) the models struggle to capture critical features in complex occlusion scenarios, and 3) the optimization objectives of person detection and ReID are in conflict in the shared feature space. To address these issues, this study proposes a novel adaptive shift and task decoupling (ASTD) method that aims to enhance the accuracy and robustness of extracting discriminative features within the region of interest. In particular, we introduce a scale-aware transformer to handle scale/pose variations and occlusions. This transformer incorporates scale-aware modulation to enhance the utilization of multiscale information and adaptive shift augmentation to learn adaptation to occlusions dynamically. In addition, we design a task decoupling mechanism to hierarchically learn independent task representations using orthogonal loss to decouple two subtasks during training. Experimental results show that ASTD achieves state-of-the-art performance on the CUHK-SYSU and PRW datasets. Our code is accessible at https://github.com/zqx951102/ASTD.