The invention provides an externally applied medicine for treating eczema and a preparation method thereof. The preparation method comprises the following steps: respectively soaking glabrous greenbrier rhizome, Fructus Forsythiae, herba schizonepetae, folium artemisiae argyi, sodium sulfate, american lopseed, licorice root and pepper in rice vinegar which is boiled and cooled; inoculating Acetobacter sp. GLYSM66 CCTCC NO:M2011238, and fermenting for different fermentation times; and blending the fermentation liquids to obtain the externally applied medicine for treating eczema. The externally applied medicine for treating eczema has strong functions of activating blood circulation to dissipate blood stasis, relaxing the tendons, activating the collaterals, dispelling wind, eliminating dampness, promoting the circulation of qi, diminishing inflammation, alleviating pain and relieving itching. The externally applied medicine for treating eczema has the advantages of obvious treatment effect and low treatment cost; and the preparation method is simple.