Urgent! Can I use my Singapore passport to go to Hong Kong?

If it is a Singapore passport, you can go to Hong Kong without a visa. You can travel to and from Hong Kong from the mainland with multiple round-trip visas just by entering the country.

But I guess what you said is that you have a China passport and have multiple visas to Singapore. In this case, if you fly from Hong Kong to Singapore (you have a ticket from Hong Kong to Singapore), you can enter Hong Kong, but if you don't go to Singapore, you can go straight back to the mainland once or twice. Basically, the third time you will be recorded as "DT", which is often called a bad record. You can't go to Hong Kong if you record DT twice, and you can't go abroad if you record DT three times.