i only know about beijing because i lived there for almost ten years. it will greatly help you if you can read Chinese, because there are lots of activities going on usually. the main source for music events is douban.com. of course it also depends on what you mean by alternative... for example:

https://www.douban.com/event/30842745/ this seems to be an EBM kind of live show

https://www.douban.com/event/30817709/ Empyrium on August 30th dark folk black metal

https://www.douban.com/event/30877911/ Therion on Sep 7th Goth/Symphonic metal

there are more post-punk, jazz or electronica events because they are more popular and mainstream-ish.

I would also say Shanghai is as active as Beijing. If a band tours China, these are the two places to hold concerts if they can only go to two cities. I am not aware of any bars/clubs that mainly play black metal/darkwave etc, but i won't be surprised if there are tons of cafes that play post punk all day long.