回复：爸妈签证快到期了,想问问如何续B2,不知道什么理由比较好,

Your parents' next B-2 will be very difficult if they extent this time, but if it's the case as you said, then go to uscis.gov and check out:

I-539, Application to Extend/Change Nonimmigrant Status

read the instructions (pay special attention to B-2 sections) that clearly state the requirements. You should also supplement documents to prove/support your reason of extension. Other additional docs may or may not help.

The most important above all is: you must provide a persuasive reason for them to accept your request.