To explore the effect mechanism of modified foshousan treating primary dysmenorrhea. Methods:The model rat of primary dysmenorrhea was established by peritoneal injection of oxytocin. The contractive amplitude and frequency of isolated uterus were observed, and the activity of uterine smooth muscle was calculated. The contents of PGF2α, MDA and the activity of GSH-PX and SOD in blood serum were determined. Results:In 2.0g/kg, 4.0g/kg of MFSS groups，the contractive frequency of isolated uterus was lower and contractive amplitude was higher than that of model group (P<0.05). The uterine muscle activity (contractive frequency × contractive amplitude) of isolated uterus was lower than that of model group (P<0.05). The contents of PGF2α and MDA in all doses of MFSS groups were lower than that of model group. The GSH-PX and SOD activity in MFSS group were higher than that of MC group. Conclusion:MFSS can obviously inhibit the activity of isolate uterus of primary dysmenorrhea and its effect mechanism is related to the decrease of PGF2α and increase of GSH-PX and SOD activity.