The adjustable fixture for a cover plate of a pickup truck includes a mounting seat, a sliding seat and a sliding block. The lower end of the mounting seat is provided with a horizontal sliding slot, and the lengthwise direction of the horizontal sliding slot is parallel to the lengthwise direction of the mounting seat. The sliding seat is engaged in and slidable in the horizontal sliding slot, and is provided with a first elongated slot. The sliding block is engaged in and slidable in the sliding seat A quick-detachable component passes through the first elongated slot and is connected with said sliding block. A tension spring is provided between the sliding block and the sliding seat. The upper end plane of the sliding block extends rearwards so as to form a connection part, and an end of the connection part bends upwards so as to form a clamping part.