Who can introduce me some popular forums about maths？?

Well, there are a few good math questions right here on Fluther. There are also plenty of mathematicians.
Here’s an interesting math discussion I started not too long ago: http://www.fluther.com/158130/if-1901111111-2902222222-3903333333-and-so-on-then-how-come-9909999999/.

Option 2: Stackexchange.

Purple math

I presume that you’ve already been to Wolfram Alpha.

Are you looking for math Q&A sites? I have gotten good answers to my questions from Math Stack Exchange and Quora

For general interest articles on math, I have found some good material on Math Plus and MAA columns. I used to use Cambridge University’s NRich site back when they had a top notch Q&A section, which they have unfortunately discontinued.

I don’t know how serious it would be – and I haven’t actually looked through it myself – but Reddit has a Math forum.