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

Here is Why You Need An Adapter

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

Bulgaria

In Bulgaria, outlets of type C and F are in use.

Type C

Type F

China

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

Type A

Type C

Type I