Hello,Mr.vinh
You have made a mistake about the place you should go for the bamboo sea. It is in Yibin City, not Longchang County. So you should head for Yibin instead of Longchang from Chengdu by train. And then take bus at the south passenger station of Yibin for half an hour to go to bamboo sea. After that, you should get from Yibin to Chongqing and then take long distance bus or train to Dazu.