Hi Orga,
Thanks for your help. Now I put desktop file in meta/gui/ folder in snap package, and app icon is also exist in app menu. App could run up from app menu, but autostart is not working. 
apps:
myapp-service:
command: bin/myapp-service
autostart: myapp-service.desktop
plugs:
- desktop
- desktop-legacy
- unity7
- x11
- home
About following description, it should copy desktop file manually by my application?
An application may put a desktop file under $SNAP_USER_DATA/.config/autostart
in order to be automatically started with the user’s desktop session. The file is
matched with a corresponding application based on the autostart property of an
app inside meta/snap.yaml. For example: