The invention discloses a combined box girder sea-crossing bridge and a construction method thereof. The sea-crossing bridge comprises bridge piers and combined box girders supported by the bridge piers; each combined box girder is defined by a high-strength concrete top slab, a superhigh performance concrete bottom plate and a pair of superhigh performance concrete webs; internal and external prestress steel beams are arranged in each combined box girder along the longitudinal direction of the bridge; the combined box girder sea-crossing bridge is simple in structure and good in durability. The construction method comprises the following steps of S1, performing construction of pier foundations, bearing platforms and the bridge piers and precasting channel girders; S2, pouring the high-strength concrete top slab to form one section of combined box girder, and tensioning the internal and external prestress steel beams in the combined box girder; S3, hoisting the combined box girder onto a temporary support; S4, mounting permanent supports and casting adjacent sections of wet jointing seams in situ; S5, demolishing the temporary supports and finishing the system conversion; S6, tensioning internal prestress steel beams in adjacent sections of wet joining seams to form a multi-section continuous girder structure; S7, finishing the bridge surface paving and accessory facility construction. The construction method is simple in process and portable and fast in construction.