Objective：To explore the relationship between physical activity，sedentary time，and irregular menstrual cycles in women of reproductive age. Methods：Conducted between April and June 2024 using convenience sampling，the cross-sectional study assessed physical activity and sedentary time through the international physical activity questionnaire - short form（IPAQ - SF），while menstrual cycles were self - reported. Associations were analyzed using logistic regression and restricted cubic spline models，with E - values calculated to assess the influence of potential unmeasured confounders. Results：A total of 1 259 participants were included， with a mean age of 25.0 ± 5.3 years. The moderate physical activity was linked to a 40% lower risk of irregular menstrual cycles compared to low activity（OR=0.60，95% CI：0.44-0.81），and high physical activity was associated with a 46% reduced risk（OR=0.54， 95% CI：0.37-0.79），after adjusting for all covariates. Meeting World Health Organization guidelines for moderate-to-vigorous activity was associated with a 36% decreased risk（OR=0.64，95% CI：0.48- 0.85）. No statistically significant association was observed between sedentary time and irregular menstrual cycles. E - value analysis suggested that unmeasured confounders would need a moderate association strength（1.8-2.1）with both exposure and outcome to negate these findings. The risk of irregular menstrual cycles began to rise when weekly total metabolic equivalents exceeded 3 185 min. Conclusion：Moderate to high physical activity levels are inversely related to irregular menstrual cycles in reproductive - aged women，though excessively high activity may increase risk.Ensuring adequate physical activity appears more crucial than merely reducing sedentary behavior.