Abstract: Objective: To explore the illness perceptions and coping behaviors regarding symptom manifestations and functional impacts of premenstrual syndrome(PMS) among young unmarried and nulliparous women. Methods: In this descriptive qualitative study, purposive sampling via an online platform was used to recruit 12 young unmarried nulliparous women aged 18-34 years with moderate-to-severe PMS for semi-structured interviews. Data were analyzed using Braun and Clarke's thematic analysis. Results: Four themes and 15 subthemes were identified:(1) symptom manifestations of PMS: physical discomfort, emotional fluctuations, compounding stress, and changes in symptoms across shifting life circumstances;(2) functional impacts: reduced work and study efficiency, additional energy required to maintain daily functioning, and interpersonal avoidance and conflict;(3) illness perceptions: limited recognition before encountering the concept, online information and peers as knowledge sources, improved self-understanding after naming the experience, and uncertainty about causes, duration and health risks;(4) coping behaviors: individualized coping attempts, limited effects and concerns about coping methods, insufficient health education and practical support, and online content as a source of both health information and stigma. Conclusion: The participants' illness perceptions of PMS are formed mainly through repeated premenstrual experiences, fragmented information and peer communication; their coping strategies are largely based on personal experience and are constrained by delayed recognition, knowledge uncertainty and insufficient external support. PMS-related health education, clinical recognition and menstrual-friendly support for young unmarried and nulliparous women should be strengthened.