If I come with a brand new laptop, how much should I pay?

You should take the receipt of your new laptop. The tax rate is 20% of the value of it.

What if I opened the box and carry it with a case, but as well bring the empty box to China? Do I have to pay for the tax too?

The laptop is not duty free either new or old, so you still need to pay tax.

But the sixth point showed :

I’m traveling to China with my Iphone, Ipad, and MacBook. Do I need to declare them and pay the tax when I enter China?

Answer: You do not need to declare and pay the tax for your personal items, such as smart phone, tablet PC, laptop, and digital camera, if within reasonable quantity (one for each). However, if you are taking another brand new phone or laptop with the outer packing and instruction book as a gift for your friend in China, you should declare the new one and pay the tax.

So do I need to pay for the tax or not? Thank you

You got it. When these items are personal use, there is no need to declare nor pay tax. No worry! I think Lisa may understand incorrectly at this point.