Objective To summarize the medication rules of prescriptions for psoriasis. Methods The external prescriptions for psoriasis in the Dictionary of Traditional Chinese Medicine Prescriptions（《中医方剂大辞典》） were collected. Based on the V 2. 0. 1 software of traditional Chinese medicine（TCM） inheritance system

data mining methods such as association rules and complex system entropy clustering were used to analyze the frequency of use of drugs

common drug pairs and combinations of Chinese medicines

core combinations of drugs and new prescription combinations of the external prescription for treating psoriasis. Results A total of 131 prescriptions for external use were selected

involving 209 kinds of drugs. There were 22 drugs with frequency larger than 10. The top 3 frequentlyused drugs were Sulphur

Calomelas and Realgar. And 48 commonly used Chinese medicine combinations were obtained

and the top 3 combinations with frequency from high to lowe Sulfur-Realgar

Shechuangzi（Fructus Cnidii）-Dafengzi（Hydnocarpus Anthelmintica Pierre）

Kushen（Radix Sophorae Flavescentis）-Shechuangzi（Fructus Cnidii）; 22 core drug combinations were obtained

including Jingjie（Herba Schizonepetae）-Shengdihuang（Radix Rehmanniae Recens）-Olibanum-Huangqin（Radix Scutellariae）-Huangbai（Cortex Phellodendri）-Huanglian（Rhizoma Coptidis）-Dahuang（Radix et Rhizoma Rhei）

etc.; Totally 11 new prescriptions were obtained by cluster

including Jingjie（Herba Schizonepetae）-Shengdihuang（Radix Rehmanniae Recens）-Olibanum-Gancao（Radix Glycyrrhizae）; Qingdai（Indigo Naturalis）-Bingpian（Borneolum Syntheticum）-Hongfen（Hydrargyri Oxydum Rubrum）-Moschus-Shancigu（Pseudobulbus Cremastrae seu Pleiones）

etc. Conclusion The external treatment of psoriasis in the Dictionary of Traditional Chinese Medicine Prescriptions is mainly based on killing ascarid

counteracting toxic substances and dispelling wind

and gradually shifts to clearing away heat and detoxifying

cooling blood and promoting blood circulation

moisturizing and relieving itching.