Unsafe acts of seafarers have been widely recognized as the direct causes of maritime accidents. Previous studies have demonstrated the significant role of unsafe acts, and provided numerous factors that affect seafarers’ unsafe acts. However, the complex relationship among factors has not been explored. Therefore, in the present study, grounded theory is employed with the objective of determining the seafarers’ unsafe acts and their contributory factors in a systematic way, and within the framework of human factor analysis and classification system (HFACS), the causation of seafarers’ unsafe acts is hierarchized. Then, rough set theory is integrated with association rule mining to explore the potential causation of seafarers’ unsafe acts. The results highlight the development of theoretical analysis framework for seafarers’ unsafe acts, and the causation of unsafe acts is presented from three perspectives: “factors→unsafe acts”, “factors self-association”, and “unsafe acts self-association”. Finally, the safety countermeasures are provided. Results from this study would help relevant stakeholders to tailor prevention measures for seafarers’ unsafe acts, and enhance the safety management level in seafarers.