'Big Bang Theory' star Johnny Galecki's burning ranch created a massive cloud of smoke over Santa Margarita, CA -- and destroyed his beloved getaway home.

TMZ obtained this photo of the brush fire burning out of control Tuesday. As we first reported ... Johnny's house went up in flames Monday night, and parts of his 300 acres of land were also scorched. No one was hurt. 

His pad sat about a half mile up the road from the entrance to the ranch in San Luis Obispo County.

According to CalFire ... they've reduced the number of firefighters from 1200 to 600 and the fire is roughly 75% contained.

We're told 2 other homes burned down in the massive fire. Johnny told us he intends to rebuild.