You Do Need power plug adapter while traveling from Thailand to China.

Here is Why You Need An Adapter

The outlet and plug type used in China are A, C and I and in Thailand are A, B, C and O. This is how they look like.

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O

China

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

Type A

Type C

Type I