In order to analyze the mechanical behavior of the new type of lining structure combined with thickened floor and large ditch on tunnel bottom in waterrich karst area, commercial infinite element software is used to analyze and calculate the excavation conditions of the central water ditch. The results show that: (1) When the primary support is set above arch foot, and the tunnel floor is not closed, meanwhile the distance between central ditch and lower bench is 15 m, the tunnel concrete reaches limit condition, and the tunnel convergence and settlement are all within allowance, but the tunnel foundation uplifting amount and plastic zone are relatively large during excavation of Grade Ⅳa and Va surrounding rocks. (2) The excavation length of central ditch every circulation affects the deformation, foundation heave and plastic zone distribution little; while the delay of tunnel closure is the main cause for the deformation, foundation heave and plastic zone distribution. (3) The maximum distance between central ditch and lower bench should be 15 m, and the excavation length of central ditch every circulation should be 5 m both in Ⅳa and Ⅴa Linings. (4) The horizontal deformation of arch foot restricted by temporary support or anchor rope + grouting reinforcement of tunnel bottom should be carried out in Ⅴa lining section.