Abstract: Objective To describe the prevalence and dynamic pain characteristics of dysmenorrhea among female medical students using a menstrual diary-based observational study. Methods From April to June 2021, female students were recruited from a medical college to complete menstrual diaries and record dysmenorrhea-related characteristics for two menstrual cycles. Dysmenorrhea severity was assessed using the visual analogue scale (VAS) (no pain:0; mild: 1-3; moderate: 4-6; severe: 7-10). A total of 323 diaries were collected, of which 303 were valid (valid response rate: 93.80%), covering 596 complete menstrual cycles. Among them, 293 participants completed diaries for two cycles and were included in the analysis of persistent dysmenorrhea. Period, point, and persistent prevalence were calculated using the number of participants as the denominator. Paired comparisons were performed using Cochran’s Q test and Bonferroni-adjusted McNemar tests. Using menstrual cycles with dysmenorrhea as the unit of analysis, the distributions of pain onset, peak timing, and pain duration across different severities were compared using the X² tests or Fisher’s exact tests. Results Among 303 female college students, 269 experienced dysmenorrhea during the first recorded cycle, and 288 had dysmenorrhea in any cycle; among 596 complete menstrual cycles recorded, 527 were associated with dysmenorrhea (88.42%), and among 293 students who completed 2 complete recorded cycles, 239 had persistent dysmenorrhea. The period, point, and persistent prevalence of dysmenorrhea among female college students were 95.05% (288/303), 88.78% (269/303), and 81.57% (239/293), respectively (Q=61.61, P<0.001). The prevalence of persistently severe dysmenorrhea was 5.12%. Among the 527 dysmenorrheic cycles, there were statistically significant differences in the distribution of pain onset time (P=0.003) and pain duration days (P<0.001) among different degrees of dysmenorrhea; mild dysmenorrhea mostly started on the first day of menstruation (52.83%), and moderate dysmenorrhea mostly started before menstruation (50.19%). Pain duration increased with severity; the proportions of cycles with pain lasting ≥4 days were 13.20%, 43.03%, and 51.56% for mild, moderate, and severe dysmenorrhea cycles, respectively. For all severity groups, mean pain scores peaked on the first day of menstruation and then decreased day by day to a stable level. Conclusion Dysmenorrhea is highly prevalent among female medical students, and differences exist across pain severity levels in onset time, duration, and dynamic changes in pain. Identification and health guidance for those with persistently moderate-to-severe dysmenorrhea should be strengthened, and medical evaluation should be conducted when necessary.