Answer: Siwu decoction is the main prescription for nourishing blood and regulating menstruation, which is mainly used for treating deficiency and stagnation of blood. In the prescription, Radix Rehmanniae Preparata nourishes yin and blood, tonify kidney and replenish essence, which is the key medicine for enriching blood, so it is the monarch medicine. Angelica sinensis can replenish blood and promote blood circulation, which is an important medicine for nourishing blood and regulating menstruation; Paeonia lactiflora nourishes blood and benefits yin; Chuanxiong promotes blood circulation and qi circulation. The compatibility of the four drugs plays the role of enriching and regulating blood. The compatibility of this prescription is characterized by matching Radix Rehmanniae Preparata and Radix Paeoniae Alba (blood medicine in blood) with pungent Angelica sinensis and Rhizoma Chuanxiong (gas medicine in blood), which is suitable for dynamic and static, enriching blood without stagnation, promoting blood without injury, warming but not dryness, nourishing but not greasy, and becomes a good recipe for enriching blood and regulating blood.