Q: My imac keep restarting after installed el capitan, help!

There is a good chance you have a 3rd party kernel extension that is not El Capitan compatible

See if you have panic files.  If so, please post at least one, or several

<http://support.apple.com/kb/ht2546>

<https://support.apple.com/en-us/HT201753>

You might also post, in another reply, the EtreCheck output

<https://discussions.apple.com/docs/DOC-6174>

You can try booting into Safe mode, and see if that changes things.  This would be a very good indication that it is a 3rd party kernel extension at fault.

<http://support.apple.com/kb/ht1564>

This happened to me.  I reset the PRAM and it worked perfectly.