Flying to Chengdu from the US, through Capital Airport

You need to go through the customs in Beijing.

Thank you for the response. Does this mean that our checked baggage will be returned to us in Beijing for customs and then re-checked for the trip to Chengdu? I am worried that the 1.5 hours between flights will be insufficient. Or do we go through customs in Beijing just with our hand luggage and then go through customs again in Chengdu with the checked luggage?

Generally speaking, your luggage can be deliveried to Chengdu directly, since you book both of the airplane tickets from Air China. You just need to go through custome in BJ and do not need to go again in Chengdu.
For more detail information, I suggest you to ask your airline company to comfirm it, so that you can arrange everthing in advance and do not worry on your journey.
Have a nice trip in Chengdu.