In this paper, the engineering characteristics and functional requirements of the superlong seacrossing tunnel on No. 3 Line of Xiamen Metro are analyzed. The concept of the civil scheme design of the superlong seacrossing tunnel is presented in terms of geologybased route selection, construction method selection and structural design. The construction method of “shield boring method + mining method + shield boring method”, the scheme of the ventilation and exhaust discharge gallery of the seacrossing section, the selection of the ventilation shaft location and the type of the structural crosssection of the tunnel are presented in detail. Comparison and contrast is made among the scheme of “large crosssection tube + small crosssection tube”, the scheme of “three tubes” and the scheme of “single large crosssection tube”, with the scheme of “large crosssection tube + small crosssection tube” adopted in the end. Furthermore, the construction technologies for the connection between the shield boring section and the mining section, the technologies for the disaster prevention and rescue of the superlong seacrossing Metro tunnel, and the structural waterproofing and drainage technologies of the seacrossing tunnel under high water pressure are studied and discussed, and some suggestions are given.