I will arrive to Chengdu airport, around 8 p.m., on September 30th. I have to reach Mianyang.

Well, the last bus to Mianyang from Shuangliu Airport is scheduled at 21:00. The ticket fare is CNY 57. You need to collect the luggage as soon as possible. See whether if you can catch it.
If not, you can stay at a hotel nearby the airport and start off in the next early morning.
The earliest train from the airport is 10:34. It takes around 1.5 hours. The ticket fare of first class seat is CNY64, second class seat is CNY54, business class seat is CNY124.
And the earliest bus is at 09:30. You can choose the way you like.
In addition, you can still exchange some money at the airport at that time. Don't worry.

Thank you for your answer. Do you know is there also train after 8 p.m.?
When does the last train from the airport go for Mianyang?

I am afraid not. The last train from the airport to Mianyang departs at 19:47, and you cannot catch it. Hurry up and try to catch the last bus.

Or, spend one night and head for Mianyang on the next morning. ;)

Thank you again Charles.