Abstract：Objective：To explore the medication rules of Wai Tai Mi Yao Fang (Arcane Essentials from the Imperial Library) in treating externally- contracted fever based on data mining. Methods： The prescriptions about externally- contracted fever in Volume Ⅰ to Volume Ⅳ of Wai Tai Mi Yao Fang were included，and the high-frequency Chinese herbs with a frequency of and over seven were extracted，and the clustering analysis and association rule analysis were carried out. Results： A total of 55 prescriptions and 86 kinds Chinese herbs were included. Glycyrrhizae Radix et Rhizoma， Scutellariae Radix， Sojae Semen Praeparatum， Zingiberis Rhizoma Recens， and Cinnamomi Ramulus were frequently used； the flavor was mainly bitter，and the nature was mainly cold and slightly cold. The cluster analysis showed that there were four core prescriptions： Glycyrrhizae Radix et Rhizoma- Ophiopogonis Radix- Paeoniae Radix Alba-Fructus Jujubae-Zingiberis Rhizoma Recens-Poria，Gypsum Fibrosum-Ephedrae Herba -Puerariae Lobatae Radix - Cinnamomi Ramulus， Bupleuri Radix- Scutellariae Radix- Rhei Radix et Rhizoma - Natrii Sulfas， Fructus Gardeniae- Sojae Semen Praeparatum. The association analysis showed twenty- eight association rules，among which (Bulbus Allii Fistulosi→Sojae Semen Praeparatum) and (Glycyrrhizae Radix et Rhizoma， Zingiberis Rhizoma Recens→Jujubae Fructus) had the highest degree of enhancement. Conclusion： This study preliminary reveals the medication rules of Wai Tai Mi Yao Fang in treating externally- contracted fever，characterized by caring for the spleen and stomach，harmonizing nutritive qi and defensive qi， releasing the flesh and clearing heat， harmonizing shaoyang and clearing yangming heat，and clearing and diffusing constraint heat.