回复：

. 1)My id in uk here is unlimited remain. How long can my parents stay here?
6 months

2)How about parents' going to hosiptal here? paying everytime?
In theory, those on a visitor visa (up to 6 months) do not qualify for free NHS. In reality, some hospitals might not check. E&A usually don't check.

3)I want to buy insurance for parnts. where should I buy? from China or uk?
In China before the trip. You might be able to buy in the UK, but it's non-standard, expensive, and rare. Make sure it covers non-UK resident travelling from abroad to the UK (as most contracts are for UK residents travelling abroad).