The invention provides a 700 DEG C reheater of an advanced ultra supercritical unit boiler. According to an existing unit reheater, along with the continuous increase of boiler parameters, existing material performance and workability will not meet requirements. The 700 DEG C reheater of the advanced ultra supercritical unit boiler comprises a reheater collection box (1) which is connected with a first heating face tube panel (3) in a welded manner through a collection box tube connector (2); the first heating face tube panel and a second heating face tube panel (4) are connected in a welded manner through a V-shaped groove; the reheater collection box is made from alloy 617B large-caliber stainless steel tubes, and the nominal diameter of the alloy 617B large-caliber stainless steel tubes is larger than 100 mm; the collection box tube connector is made from alloy 617B5 small-caliber stainless steel tubes, and the nominal diameter of the alloy 617B5 small-caliber stainless steel tubes is smaller than 100 mm; and the reheater heating face tube panels are made from Sanicro25 stainless steel. The 700 DEG C reheater of the advanced ultra supercritical unit boiler is applied to increase of the main steam temperature and reheated steam temperature of a thermal power unit.