While working in China and being asked to help by a Chinese man, however, you are too busy to help him, what could you say to refuse him?

Learn to refuse someone in Chinese. First of all, you should feel sorry that you are not able to offer help.
Example sentence:
很抱歉，我帮不了你。
Hěn bàoqiàn, wǒ bāng bùliǎo nǐ.
I am sorry, but I could not help you.

Then, you'd better explain why you could not offer help.
因为我现在也很忙。
Yīnwèi wǒ xiànzài yě hěn máng.
Because I am so busy now.

And in the end, you should promise to offer help in the future.
下次只要我有时间，一定会帮你的。
Xià cì zhǐyào wǒ yǒu shíjiān, yīdìng huì bāng nǐ de.
I promise to help as long as I have time!