There is a ring road between Chengdu and Jiuzhaigou. When you arrive at Chengdu you can take a bus to Jiuzhaigou directly.
The ring road of Jiuzhaigou is an important tour route of Sichuan, 913km long and with two directions (east and west). So there are two routes constituting the round- trip between Chengdu and Jiuzhaigou.

West route: approximately 430 km
Chengdu–Dujiangyan–Wenchuan–Maoxian–Songpan–Chuanzhu temple–Jiuzhaigou
East route: about 520 km
Chengdu–Guanghan–Deyang–Mianyang–Jiangyou–Pingwu–Jiuzhaigou county–Jiuzhaigou

Both of the two routes take 9-10 hours. The highway from Chendu to Mianyang Dujiangyan and Jiangyou is smooth and wide, which makes the winter tour possible. In the summer rainy season, landslides sometimes happen here, however, the local government does a good job protecting for the road. Generally the problems are solved after several hours.

The east route highway is longer than that of the west one. The total length is approximately 170km. There are direct buses from Mianyang to Jiuzhaigou in the east route. And the west route goes along Ming river passing 5 towns where you will see the traditional culture of Zhang people and Qiang people, the destroyed vegetation and the well preserved nature resources.

The highway from Chuanzhu temple to Jiuzhaigou was opened in September 2003 which is the most important route to Jiuzhaigou. It is safe, comfortable and brings a lot of enjoyment to the tourists, starting at the Chuanzhu temple town and ending at the entrance of Jiuzhaigou (about 94.14km, an 1.5 hours journey)
In Chengdu (Xintianmen bus station) air-conditioned buses go to Jiuzhaigou at 8:00 AM every day, costing about RMB100. Starting from Mianyang or Leshan you can also take the bus to Jiuzhaigou.