(1) Background: Currently, Taiwan has adopted a “person-centered” approach to Advance Care, including Hospice Palliative Care and Advance Decisions, both of which are intended to enhance the right of individuals to choose their own end-of-life care; however, it is extremely challenging and difficult to implement the principle of autonomy for psychiatric patients. (2) Methodology: The aim in this study is to investigate the factors affecting the intention of day ward patients to sign up for hospice and palliative care by using the questionnaire content of the “Survey on Knowledge, Attitude Toward, Experience, and Behavior Intention to Sign Up for Hospice and Palliative Care”. A cross-sectional design compliant with STROBE (Strengthening the Reporting of Observational Studies in Epidemiology) was employed. An independent samplet-test, Pearson’s correlation analysis, and stepwise regression analysis were used to determine the factors influencing the intention of psychiatric patients to sign up for advanced care planning. (3) Results: The relationships between knowledge of and attitude toward advanced care planning, knowledge of and behavior intention to sign up for advanced care planning, and attitude toward and behavior intention to sign up for advanced care planning were all positive (p-value < 0.001). The final three most relevant indicators were attitude toward hospice and palliative care, hospitalization of family members during the previous five years, and death of a close friend within the previous five years. (4) Conclusions: The results of the study show that the hospice and palliative care attitude and past experience of psychiatric patients affect the intention to sign up, reminding us that psychiatric patients are at an increased risk of decision-making disability as their illness progresses and that, in addition to initiating the discussion of Advance Care Planning as soon as possible, it may be an opportune time for medical professionals to actively promote Advance Care Planning among their patients.