A foldable host unit case for a computer with joint mechanisms includes four holding posts and the upper ends of the holding posts connect with a neighboring holding post with first foldable connecting rods and second foldable connecting rods respectively. The lower ends of the holding posts are joined to four ends of a cross connecting rod set such that the holding posts are capable of being extended to constitute a firm structure. Hence, the host unit case is capable of being extended conveniently and quickly and being folded conveniently to reduce the volume thereof during being packaged.