A knockdown water-cooling unit latch device structure includes a latch device assembly having multiple latch members. The latch members are correspondingly assembled with each other around a water-cooling unit. The latch members are connected with each other via at least one connection member. Alternatively, the latch members are directly assembled with each other by means of engagement or lap joint. The latch members of the latch device assembly are assembled with the water-cooling unit so that when the latch device assembly is assembled with the water-cooling unit, the latch members will not interfere with the water-cooling unit.