Scott Darroch was caught with 150 plants worth more than £20,000 in his isolated farmhouse in Skelmorlie, Ayrshire, in April 2014.
In January, Sheriff Shirley Foran jailed the former Royal Engineer - who served in Iraq, Afghanistan and Kosovo - after he admitted producing and being concerned in supplying cannabis.
Kilmarnock Sheriff Court had heard Darroch, 42, suffered from severe post-traumatic stress disorder (PTSD) and often checked streets for roadside bombs, believing he was still on active service.
He “self-medicated” with the drugs, which he shared with fellow veterans who contributed to cultivation costs.
The Criminal Appeal Court has reduced his sentence to 18 months, ruling that although custody was necessary there were “substantial mitigating factors”, including a “lack of commercial enterprise”.
Lord Justice Clerk, Lord Carloway, sitting with Lord Malcolm, said: “The court has considerable sympathy for the appellant and his current predicament following the traumatic experiences which he has had in the service of his country abroad.”
Diarmid Bruce, Darroch’s solicitor, said: “There was an unusual background, including PTSD from Mr Darroch’s service in Iraq, and I thought the sentence was excessive.”