Long-acting antipsychotic injections require that psychiatric nurses choose the proper injection site and technique to avoid harming patients. This research conducted a mixed-method study to examine the long-acting injectable antipsychotic (LAI) knowledge, practice, and administrative barriers in a sample of 269 psychiatric nurses from 3 public psychiatric hospitals in Taiwan. Self-report questionnaires showed female nurses exhibiting higher scores and older nurses demonstrating more knowledge. The dorsogluteal (DG) site was the most widely used for injections, with 57.6% of nurses using the Z-track method. Qualitative data analysis was performed on 20 psychiatric nurses who chose the DG site as their preferred injection site. There were two key themes. The first was a gap between the nurses’ knowledge of LAI administration and their actual practice. The second needed more confidence and training in using the ventrogluteal injection site. These results highlight the need for continued education and training to improve LAI practice among psychiatric nurses.