The invention provides a combined external medicine for treating prostate diseases and a method using the same. The combined external medicine includes a pre-immune composition comprising multi-immune powder and a noble bottle tree decoction, a lesion active composition comprising the multi-immune powder and a first medical decoction prepared from a first Chinese medical composition for treating prostate diseases, and a double rib active composition comprising the multi-immune powder and a second medical decoction prepared from a second Chinese medical composition for conditioning liver and kidney. The method includes applying the pre-immune composition on skin of lesion part and skin of the left and right ribs; waiting for 5-10 minutes; applying the lesion active composition and the double rib active composition on the skin of the lesion part and skin of the left and right ribs; and airing for 10-40 minutes.