I am a Chinese from Hong Kong, what business I can open here in Thailand ..?

First off, you should know that there are many restrictions for foreigners who operate a business in Thailand - many of these restrictions are frustrating and complex and require the help of a lawyer and/or a trustworthy and reliable Thai representative (many foreigners use the face of their Thai girlfriend, wife or friend to run their business). Before you figure out what business you will operate, it is important to understand the limitations and restrictions you will be up against.

If you are still happy to proceed, figure out how much money you are willing to invest and see what it can buy! Markets, motorcycle rental and accommodation may be some ideas.

Out of curiosity, do you have a budget or you just prefer Thailand for personal reasons?
I think opening a takeaway/restaurant is very good, I would do it. I'm based in the UK and where I study, there are so many Chinese students and there's a lot of Hong Kongers opening small restaurants near the university and it is always busy, they make so much money but have to put up with a lot of hard work.

I say open anything that will attract Chinese tourists but I heard Thailand is very cheap so not sure how much you can make in comparison to if you were to open up similar business in HK.

But I don't have enough money to open
What else I can open with limit money

If you obtain a Thai citizen partner, you can open virtually any biz in Thailand. However, there are strict limits on what jobs you can do... other than be an executive/part-owner.... and you must have a Thai Work Permit to do anything, and I mean anything.

Likely a push-cart food vendor takes little money to start, about 5000THB, but you will have big problems getting the food to sell and even larger problems of finding an open space on the sidewalk to operate (you can go out in the country and set up under a highway underpass with less problems except police).

If you are asking, it is very unlikely you should pursue this idea. Consult a good lawyer first.

Work at a factory ,they pay better than in China and they give you free housing and a free car if you remain in good standing for 6 months ,that's why so many Thai people do it.

It's hard to open a business with limited money. Try to obtain a loan first. Consider opening a Chinese restaurant.