Wàimiàn xià zhe piáopō dàyǔ.
外面下着瓢泼大雨。
It's raining cats and dogs outside.
Notes:
外面(wàimiàn): out door
下着(xià zhe): be raining
瓢泼大雨(piáopō dàyǔ): extreamly heavy rain
Example:
A:外面下着瓢泼大雨。
Wàimiàn xià zhe piáopō dàyǔ.
It's raining cats and dogs outside.
B:我有伞。
Wǒ yǒu sǎn.
I have an umbrella.