I guess this topic fits better to the altcoin section. What particular problem do you see with merge-mined coins that have different block times? I see none. The prev hash points to the previous block hash of the chain in question (Huntercoin in your example), just as always. Why do you think it should be different?
Maybe I got confused about parent block and bitcoin block. I tried to understand, a parent block is not eqaul to a bitcoin block, In auxpow block, the previous parent block hash points to the last parent block not bitcoin block. I'm right ?
I'm still not 100% sure what you mean here. Your merge-mined block (say HUC) has, very roughly speaking, two pointers to other blocks: The previous HUC block, which forms the HUC blockchain and is in every aspect just like things work in Bitcoin or other blockchains. And it points to a Bitcoin block header, which forms the auxpow - but note that this header must not actually be a fully valid Bitcoin block (nor must it be part of the main Bitcoin blockchain).
If you happen to find a "Bitcoin block" whose hash is good enough for HUC's difficulty but not Bitcoin's, you can still use it as auxpow and have found a HUC block (but not a Bitcoin block). The other way round, if you find a block whose difficulty is high enough for Bitcoin, you have found blocks for HUC and BTC at the same time. (Assuming that Bitcoin's difficulty is actually higher than that of HUC.)
