Background: Depression is significant mental health issue among university students and can decrease their ability to succeed academically. While various factors associated with mental health problems have been identified, researchers are increasingly exploring the role of positive mental health, such as character strengths and inner strengths, in relation to mental health problems. Objective: This study aims to build upon previous research by examining the influence of positive mental health on the mediation model of depression among students enrolled at Chiang Mai University. Method/Design: This study will employ an observational and longitudinal study that collects data from undergraduate students at Chiang Mai University during the 2023–2024 academic year. The primary outcome of this study will be depression. In the mediation models, insecure attachment and negative family climate will serve as predictors, while borderline personality symptoms will be the mediator. Positive mental health, including character strengths, inner strengths, and resilience, will be tested for moderation effects on the mediation models. Data will be collected at three intervals, with a three-month gap between each collection period. Discussion and Conclusion: This study will provide insights into the positive and negative mental health outcomes of university students in Chiang Mai. Through comprehensive analysis, this study intends to offer valuable insights into both positive and negative mental health outcomes among university students in Chiang Mai. Moreover, by employing a longitudinal approach, it aims to establish a more robust understanding of the causal relationships between positive mental health, predictors, mediators, and depression. The study’s limitations will also be discussed.