Do i need visa if my flight transit at 2 airport in China?

Actually, this is very simple. All you have to do is to calculate the entire time starting from your scheduled time of landing in Shenzhen until your scheduled time of departure from Beijing. If that entire time is shorter than 24 hours, you are eligible for 24 hour visa free transit and you do not need any kind of visa. However, if your entire transit time is longer than 24 hours, you will need a transit visa obtained in advance.

Under the assumption that your entire transit time in China is shorter than 24 hours, you will print out your flight tickets and have them at hand for easier handling and to speed up all the formalities because you will be asked for them already in Hanoi at the time of check-in.

When you land in Shenzhen, the first thing you will do is to approach 24/144 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to cross the immigration because your next flight to Beijing is a domestic one. This application is free of any charge and here you will present your flight tickets. After the permit is granted you will proceed through immigration and then to perform the formalities for your next flight. When you land in Beijing you will just queue for exit immigration and head to the gate of your flight for Paris.

This is it.