When I flush toilet it backs up at the drain waste vent. Just had septic emptied last week and didn't fix problem, any suggestion would help.

Assuming that the laterial lines are clear from the septic and it has not already merely filled up with water.

There must be a clog or partial blockage in the waste line going to the septic, somewhere past the drain vent.

Of course if the laterial lines from the septic or the inlet to them in the tank is blocked or clogged or bad, then the water and waste gong into the tank has no where to go

They checked the lines when they came out to empty septic, and they were OK. I am thinking it is a blockage between vent and septic, just don't know best way to deal with that.

If your entire house isn't blocked and backing up we can look other places. My guess would be the branch from the toilet. This can be snaked from one of two locations.
1. Either from a lavatory or a toilet roof vent, **OR**
2. Pull the toilet and snake from there.
Good luck and let me know how you make out, Tom