Abstract:Limited rock cover for subsea tunnel is put forward and explained in detail. Besides,the differences and relations among limited rock cover,minimum rock cover and rational rock cover are discussed. What¢s more,the way of determining limited rock cover under assistant construction methods is given. Because the longitudinal section selection of subsea tunnel is directly related to four aspects such as safety,cost,technique application and environment influence,based on the principle of limited rock cover,the selection of longitudinal section is discussed further. In view of risk coefficient,safety risk,cost risk,technique application risk and environment influence risk are considered during the assessment process of cross-section selection. Furthermore,corresponding risk factors are found. Combined with analytic hierarchy process(AHP) method,a set of assessment system for subsea tunnel longitudinal section is established. Finally,this assessment system is successfully applied to Xiamen Xiang¢an subsea tunnel;three supposed longitudinal sections are evaluated. Afterwards,the advantages and disadvantages of every cross-section are given,which can provide reference to other subsea tunnels in planning.