There is growing recognition of advance directives (AD) in dementia, however a gap remains between their perceived importance and actual implementation. This emphasizes the need to enhance healthcare professionals' support for people with dementia (PWD) with regards to making AD. This study aimed to investigate the competence, difficulties, and support needs of nurses in assisting institutionalized PWD to make AD. This mixed methods study included both quantitative and qualitative data analysis. Purposive sampling was used to recruit potential participants, and a self-developed questionnaire was used for data collection. Data were analyzed using descriptive statistics and t-test for quantitative information, and content analysis for qualitative information. A total of 188 questionnaires from 19 institutions were completed. The scores of knowledge, attitudes and skills in assisting PWD with making AD were 33.9, 39.4 and 39.5, respectively. The nurses stated that the difficulties in assisting PWD with making AD were a lack of relevant expertise, understanding the AD, and that they were unsure of how to handle conflicting viewpoints within the family. More in-service training and online courses on dementia and AD were required, as well as the creation of health education pamphlets and patient decision aids to help PWD and their families make decisions about AD. The results of this study can serve as a reference for understanding the competencies, improvements, needed support and in-service education of nurses in an institutional setting to assist PWD with making AD. We recommend the promotion of AD through related leading associations and their institutions.