Does a Pakistani passport holder require tranist visa for China

No, you don't need any kind of visa because your itinerary is eligible for 24 hour visa free transit. You will print out your flight tickets and have those printouts with you. This is important detail because I assume that you will travel on separate tickets since you will have flights from both terminals of Shanghai airport. You will also declare at check-in in Singapore that you will travel to US via Shanghai using 24 hour visa free transit. After a short inspection of your flight tickets, you will be allowed to board the plane.

Upon your arrival in Shanghai, you will first approach 24 hour visa free transit counter where you will apply for a temporary entry permit because you will have to collect your luggage, change the terminals and also perform a recheck for your onward flight to US. After you have cleared all the formalities, you will proceed to the Terminal 2, perform the formalities and wait at the gate for your flight out of Shanghai.