I'm on a one year multiple-entry non immigrant-O visa ,I entered Thailand on June 25th and I can stay until Sept,22nd

I plan to make a visa run to Laos before that date, Do I have to apply for a Re-entry permit before ?

Thank you

Multi entry means you can enter/exit at will and have no need for a re entry permit (as long as it is within the ending date of your visa stamp).  When you next enter you will receive another 90 day stamp (the old one will end when you exit).

It is only when you have a single entry or an extension of stay that you need a re entry permit.