The invention relates to a dry cabin for maintaining an underwater joint of a jacket and a mounting method of the dry cabin. The dry cabin comprises a cabin body and an access way communicated with an inner cavity of the cabin body. The cabin body comprises at least two split cabin sheets capable of being separated from each other, and the split cabin sheets are spliced to form the cabin body. Splicing lines of the split cabin sheets are in sealing fit, and two first sleeve holes used for matching circumferential rings of the outer walls of the two ends of a pile leg respectively and a second sleeve hole used for matching the circumferential ring of the outer wall of the jacket are formed in the splicing lines. The first sleeve holes are in sealing fit with the pile leg, and the second sleeve hole is in sealing fit with the jacket. The access way upwards extends out of a water surface. The dry cabin can be mounted underwater in a spliced mode, and the underwater joint is isolated from the outside, so that the sealing performance of the internal part and the external part of the cabin body is guaranteed. After water in the cabin body is pumped out, the water can enter the cabin body from the access way to maintain the underwater joint, and maintenance is easier. Meanwhile, the interior of the cabin is in a normal pressure state, so that the repairability of the underwater joint is improved, the jacket is continuously utilized, and cost is reduced.