863211老师, Good morning! Have a nice day:))

according to Tax Law, they can be dependent if they can stay 183 days in one year. 用父母探亲报税的话必须要呆够6个月, but not 180 days.

dependent from 1040 instruction. Because your parents are not Americans or permanent residents, you will have to find the definition of "resident for tax purpose". This is how it defined:
http://www.irs.gov/taxtopics/tc851.html