Objective To explore the prevention and treatment effect of Caryopteris incana decoction on primary dysmenorrhea rats with cold coagulation and blood stasis syndrome. Methods Sixty healthy SPF-grade SD female rats were randomly divided into normal group, model group, ibuprofen group, high-dose group of Caryopteris incana, medium-dose group of Caryopteris incana and low-dose group of Caryopteris incana, with 10 rats in each group. Except for the normal group, the other groups were treated with cold stimulation combined with estradiol benzoate and oxytocin to establish a rat model of primary dysmenorrhea with cold coagulation and blood stasis syndrome. On the 5 th day of modeling, the rats were given intragastric administration for 10 days. The symptoms of rats were observed and recorded. The writhing response of rats was measured. The hemorheological indexes of rats were measured. The levels of plasma thromboxane B2 (TXB2), 6-keto-prostaglandin F1α (6-Keto-PGF1α), estradiol (E2) and progesterone (PROG) in serum of rats were measured. The levels of prostaglandin F2α (PGF2α), prostaglandin E2 (PGE2), NO and Ca2+ ^{in uterine} tissues of rats were measured. The organ indexes of uterus and ovary were calculated and the histopathological changes were observed. Results Compared with the model group, the symptoms of the rats in the high and middle dose groups were significantly improved. Each dose group of Caryopteris incana decoction could significantly prolong the latency time of writhing reaction (P＜0.01) and reduce the number of writhing times (P＜0.01). The high and medium dose groups of Caryopteris incana decoction could significantly reduce plasma viscosity, hematocrit of rats, and the whole blood viscosity at high, medium and low shear rates (P＜0.01). The high and medium dose groups of Caryopteris incana decoction could significantly reduce the levels of TXB2 and E2 and E2/PROG ratio in serum of rats and increase the levels of 6-Keto-PGF1α and PROG in serum of rats. The high and medium dose groups of Caryopteris incana decoction could significantly reduce the uterine index and ovarian index of rats (P＜0.01, P＜0.05). The high and medium dose groups of Caryopteris incana decoction significantly improved the morphological lesions of uterus and ovary. The high and medium dose groups of Caryopteris incana decoction could significantly reduce the levels of PGF2α, Ca2+ ^and PGF2α/PGE2 ratio in uterine tissue of rats (P＜0.01, P＜0.05) and and increase the levels of PGE2 and NO in uterine tissue of rats (P＜0.01). Conclusion Caryopteris incana decoction can effectively improve the clinical symptoms of primary dysmenorrhea rats with cold coagulation and blood stasis syndrome, and has a good control effect. Its mechanism may be correlated with the levels of TXB2, 6-Keto-PGF1α, E2 and PROG in serum and PGF2α, PGE2, NO and Ca2+ ^{in uterine tissue.}