To analyze the medication features and regularity of prescriptions of Chinese medicine in treating patients with dementia based on ancient medical records. In the article, we retrieved the ancient medical records related to the treatment of dementia (from the Han Dynasty to the late Qing period) in Chinese Medical Classics, Chinese Ancient Medical Books and digital library, and then set up a medical records normalized database. The medication features and prescription rules for dementia were analyzed by frequency statistics and association rules (Apriori algorithm, improved mutual information algorithm and complex system entropy clustering). Finally, a total of 156 prescriptions were selected, involving 123 Chinese herbs, with a total frequency of 11 747 for the herbs, and 8 core prescriptions were mined. After the association rules between the frequency and prescriptions for the treatment of dementia were determined, we found that the most commonly used herbs included Fuling (Poria), Yuanzhi (Polygalae Radix), Renshen (Ginseng Radix et Rhizoma), Shichangpu (Acori Tatarinowii Rhizoma), Gancao (Glycyrrhizae Radix et Rhizoma), Danggui (Angelicae Sinensis Radix), Maidong (Ophiopogonis Radix), Baizhu (Bletillae Rhizoma), Dihuang (Rehmanniae Radix) and Ganjiang (Zingiberis Rhizoma); the frequently-used drugs compatibility was mainly for tonifying Qi-blood, regulating Yin and Yang and inducing resuscitation. The drugs were mainly of warm nature and sweet (mild) flavor, and the channel tropism of drugs mainly distributed to the heart, liver, spleen and kidney. The core prescriptions were composed of Renshen (Ginseng Radix et Rhizoma), Fuling (Poria), Yuanzhi (Polygalae Radix), Shichangpu (Acori Tatarinowii Rhizoma), and Baizhu(Bletillae Rhizoma). In conclusion, high frequency herbs and core prescriptions reflect the prescriptions by ancient physicians mainly focus on Qi-replenishing, spleen-invigorating and heart-nourishing, but also reflect the prescription rules of nourishing Yin, enriching blood, eliminating phlegm and warming Yang for the treatment of dementia. The medication features and prescription rules for the treatment of dementia obtained by association rules are useful to guide the clinical practice of Chinese medicine in treatment of dementia.