A host unit case with foldable front and rear walls and lower cover for a computer includes a front wall, a rear wall, an upper cover and a lower cover. The lower end of the front wall connects with the front end of the lower cover. The lower end of rear wall pivotally connects with the rear end of the lower cover. Hence, the lower cover, the front wall and the rear wall are unfolded to allow the lower cover engaging with upper end of the front wall and upper end of the rear wall for the host unit case being set up as a complete form quickly and being folded parts thereof to reduce the volume thereof conveniently.