If you've not uploaded a custom skin, you'll have either the classic "Steve" skin, or the newer "Alex" skin, which features slimmer arms. You can choose between the two model types, download the default models, or download the templates all at Minecraft.net/profile. Your default is randomly based on your account ID, is shown when you don't have a connection to the skin servers, and cannot be changed.
Slim-armed skins are only fully supported in version 1.8 and higher; if you have a slim-armed skin selected in a version prior, you will see black pixels on the arms.

如果您从未上传过自定义皮肤，那么你的默认皮肤可能是经典的“Steve”，也可能是新的，具有较细上臂的“Alex”。你可以在两种模型之间切换，下载默认模型，或在Minecraft.net/profile下载所有皮肤模板。你的默认皮肤与你的账号ID决定的随机数有关。如果你无法连接到皮肤服务器，就会显示此默认皮肤。默认皮肤无法修改成另一种。细胳膊皮肤仅在版本1.8之后才提供完全支持。若你已经应用细胳膊皮肤，那么在旧版本中，与粗胳膊相比缺失的像素将用黑色填充。