The difficulty and risk of construction are high if the shallow metro tunnel under trench is constructed with the mining method. So how to ensure the safety of tunnel and trench during the construction becomes a matter of great concern. The tunnel undercrossing the trench between Xiaolingwei station and Zhonglingjie station on metro line 2 in Nanjing is presented in this paper. The empirical formula and the finite element method are used to determine the suitable support. And the support plays an important role in the construction of tunnel. These practical experiences can provide references to the future similar constructions.