Mower belt keep falling off - Husqvarna 26 Hp 54" Deck Zero Turn Riding Mower

go see your dealer that sold it to you . they did not put it together properly

Either the belt tension is off or one of the pulleys down there is not properly aligned and the belt falls off. Or maybe the belt has stretched and needs to be replaced.

Hi Kenneth:
There's a good chance that one of the pulleys has a problem.
With the engine OFF, lie so that you can see the belt and pulleys. 1) Try to move the cutter pulleys while holding the blade. Any movement will indicate a stripped or loose connection.
2) Have a helper move the engage lever back and forth so that the belt pulls against the pulleys. Look for any unwanted movement like wobble in the pulleys.
3) Check the idler pulleys to make sure the bearings are OK.

I have the same mower. Check the idler pulley that tension the belt when you pull up on the handle. Make sure it turns freely.when there is no tension on the belt. Depending on how long you've had this mower, your belt could have stretched and needs to be replaced. I hope this helps.

look at the pulleys for play also how is the spring tension ?

Either the belt is stretched out and needs to be replaced or there is an idler/tensioner pulley that may be weak and not putting enough pressure on the belt.

You need to inspect the pullies holding the belt, and the bearings that run the pullies. If they are bad they will jump the belt.

Ensure the pulleys are exactly in line.

you have belt guides near your pulleys that helps to keep the belt on during no engagement
you need to check to see if they are missing or bent and adjust as close to the pulley and belt as possible without interfereing with the belt and pulleys while engaged.IE: no rubbing against the pulley or belt while engaged or moving.

ck belt for wear ck between deck and frame
for so thing around clutch shaft or shaft out place.