You Do Need power plug adapter while traveling from Cocos (Keeling) Islands to China.

Here is Why You Need An Adapter

The outlet and plug type used in China are A, C and I and in Cocos (Keeling) Islands is I. This is how they look like.

Cocos (Keeling) Islands

In Cocos (Keeling) Islands, outlets of type I are in use.

Type I

China

In China, outlets of type A, C and I are in use.

Type A

Type C

Type I