A cap for a drain hose and a plurality of bottles with different opening sizes is provided. The cap includes a main body and an extension body. The main body includes a main sleeve, a sleeve for drain hose and a leakage-proof part, wherein the main sleeve is for engaging with a first bottle, the sleeve for drain hose is for engaging with the drain hose, and the leakage-proof part is for closely sealing the opening of the first bottle. The extension body is formed integrally with the main body and extends outwardly from the main body, the extension body including an extension sleeve for engaging with a second bottle, the opening size of the second bottle being larger than the opening size of the first bottle.