According to the future development of high dam navigation structures of tunnel design,combined with the calculation formula of the critical speed of the ship,taking Xiluodu tunnel as an example,the tunnel economy is analysed.We put forward that the analysis of tunnel section economic should be combined with the one-way time through the navigation structure to determine the cross section coefficient of tunnel water,and the best principle is to make the ship through the tunnel of time and one-way time through the navigation structure is basically the same,so the two match.The design speed of the ship is calculated through the tunnel length and travel time,and the cross section coefficient and the design of section size are get,so as to the best balance between transit capacity and engineering investment is achieved.