I am using android.support.design.widget.FloatingActionButton from the new Design Support library 22.2.0, in my layout file. It is working fine but it shows a square button below API level 17
This is known by Google and will be fix soon in the next version of the Design Support Library, see this issue: FAB Appears as a square on API 16. It should only concern API 16.
The workaround, proposed in the same thread, is to add app:borderWidth="0dp" to your FAB.
Finally the issue has been fixed in the new release
compile 'com.android.support:design:22.2.1'
this will solve the square layout issue in API 16. FAB performs well if you place it inside Coordinate Layout. Check out https://code.google.com/p/android/issues/list?can=1&q=label:Version-22.2.1 for issues and fixes in new version.
Released version 22.2.1 This release contains bug fixes related to FAB
https://code.google.com/p/android/issues/detail?id=175067
Change your gradle
compile 'com.android.support:design:22.2.1'
Install support repository revision 16. So you can avoid the workaround [app:borderWidth="0dp"]
If it helps, I solved my issue by removing
android:background
It was a remnant from the previous FAB library I used. The design library's gets the color automatically from colorAccent.
Add the following attribute to the xml floating button
app:borderWidth="0dp"
use different library projects which supports floating action button below API level 17 . you can find easily here .plz refer this .
来源：https://stackoverflow.com/questions/30635908/floatingactionbutton-square-button-below-api-level-17