24 Hour transit

Dulari, you didn't offer any itinerary, so it is quite hard to present a definitive answer. OK, I will assume that you will travel on the following itinerary:

US - Beijing or Shanghai (not both) - Sri Lanka

If I'm right, you are definitively eligible for 24 hour visa free transit and you don't need any kind of visa.

You will print out your flight tickets and you will take those printouts with you. Also, you will declare at check-in in US that you will travel to Sri Lanka via Shanghai or Beijing using 24 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.

Upon your arrival in Shanghai or Beijing, you will immediately approach 24 hour visa free transit counter where you will apply for a temporary entry permit to be allowed to exit the airport or to collect your luggage and perform a check-in for your flight for Sri Lanka in case that you fly on separate tickets. In the case if you are flying on a single ticket, you are advised to book your accommodation in Shanghai or Beijing in advance and to print out a confirmation of your accommodation booking. This will increase your chances to be granted a temporary entry permit.