You can get the administrative boundary via the Overpass API. The corresponding query is:
relation
["boundary"="administrative"]
["admin_level"="2"]
["name:en"="People's Republic of China"];
(._;>;);
out body;
In order to learn more about the specified tags take a look at the OSM wiki. Especially on relations with the administrative_boundary tag.
The returned XML file has to be converted to a shapefile afterwards.