A locking structure for a truck bed cover includes a side rail and a fixing assembly. The side rail is detachably installed on the truck bed. One side of the side rail is attached to the truck bed, and the other side of the side rail is provided with a first limiting member. The fixing assembly includes an inner clamping block, an outer clamping block and a fastener. The inner clamping block and the outer clamping block lock the truck bed and the side rail by the fastener. The surface attached to the side rail of the outer clamping block is provided with a second limiting member. The first limiting member and the second limiting member are engaged to limit the movement of the side rail. The new locking structure is convenient and quick to install, disassemble and adjust, has good stability, and can be used repeatedly.