Provided is an opening-and-closing mechanism for horizontal-type pressure cylinder hoops. The opening-and-closing mechanism for the horizontal-type pressure cylinder hoops comprises a frame which is of a quadrangular structure, wherein a Y upward hoop, a Y downward hoop, an X towards-the-left hoop and an X towards-the-right hoop are installed on an inner ring of the frame, a round structure is formed by the Y upward hoop, the Y downward hoop, the X towards-the-left hoop and the X towards-the-right hoop, an opening-and-closing oil cylinder is installed on each hoop, opening-and-closing oil cylinders are fixed on the frame, a hanging follow-up device is arranged on the top of the frame, the X towards-the-left hoop and the X towards-the-right hoop are respectively provided with a connecting frame, a hanging frame installed on the lower portion of the hanging follow-up device is fastened with each connecting frame through fastening parts, a supporting device is arranged at the bottom of the Y downward hoop, the joints of every two adjacent hoops are connected through bolts, and the bolts are screwed down through nuts. According to the opening-and-closing mechanism for the horizontal-type pressure cylinder hoops, the oil cylinders serve as executive components to accomplish opening and closing of the hoops. The opening-and-closing mechanism for the horizontal-type pressure cylinder hoops has the advantages of being simple and compact in structure, simple and reliable in the opening process and the closing process of the hoops, and capable of easily achieving automation of the whole process, can well accomplish opening and closing of the hoops, and is an indispensable constituent part of a horizontal-type deep sea environment test device.