3 Reasons a Chainsaw Chain Keeps Coming Off

Chain tension is too loose A chainsaw chain should not sag on the bottom of the guide bar. You should be able to pull it up and see about 1/8 inch (3mm) of the drive links in the bar groove. Tightening the chain fixes 80% of derailments.

The guide bar is worn or dirty Uneven bar rails, especially a ridge on the inside edge, let the chain walk off. Flip the bar regularly and replace it when worn. Also clear sawdust from the bar groove.

Sprockets are worn or the chain is dull A worn sprocket nose or drive sprocket lets the chain slip. A dull chain bounces and can pop off the bar. Keep the chain sharp and replace worn sprockets.