Does people from Hong Kong who work in US and have a US citizenship need to pay tax to both US and Hong Kong?

Basically if you earn money in / from HK or overseas then you have the choice of paying either US or overseas taxes. If you only earn income in the US then you just pay US taxes.

US taxes all US citizens, legal permanent residents, anyone else living, working, investing, or having other US income ("US persons") on worldwide income from all sources worldwide - regardless where they live! As a US citizen, even if you return to live in HK & work there, you remain subject to US tax laws and have to file annual income tax returns & pay US taxes on your HK income (and everything else). US is one of the very few countries (only major or "developed" country to do this). So, where do you live, where do you work, what income do you have from what sources where? If you are living & working in the US, you are not subject to HK taxes (not residing in HK), but you are subject to US taxes. If you returned to HK, lived & worked there, you become subject to HK taxes, and remain subject to US taxes. However, there is a Foreign Earned Income Exclusion (no income tax on first $90-odd thousand from wages/salary - number changes annually), and you deduct HK taxes paid from your US income tax bill. Net result is slightly lower than regular US income tax rate due to income exclusion. If your total earned income + housing allowance is less than the exclusion, then only HK taxes at HK rates would be paid. BEWARE! New financial reporting requirements for reporting all foreign bank & other financial accounts! The IRS is on the rampage tripping up foreigners in US, dual nationals, and US citizens abroad. Involves major felonies and huge fines. The US is broke & they want your money - all of it & then some more. You may sorely regret your choice to move to US & get US residency/citizenship, because now you are trapped in the US tax net. Have fun.