Do i need to apply visa for china?

Your itinerary is definitely eligible for 24 hour visa free transit and you don't need any kind of visa.

You will print out your flight tickets and have those printouts with you. Also, you will declare at check-in in Frankfurt that you will travel to Kathmandu via Chengdu using 24 hour visa free transit. After a short inspection of your flight tickets, you will be allowed to board the plane. I assume that you are flying on a single ticket by Air China, but you should ask at check-in whether your luggage will be tagged all the way to Kathmandu, just for a case.

Upon arrival in Chengdu, you will just follow the sign "international transit/transfer" and if necessary, you will apply for a temporary entry permit. Also, you will clear immigration and perform other formalities, but nothing to take too much car about. In reality, after you have accomplished the formalities, you will just walk to your boarding gate and wait for your onward flight to Kathmandu.

Thank you very much for the information.