The lattice girder is widely used in the construction of composite lining of mountaincrossing tunnel. In order to study the mechanical effect of 8shaped diagonal bar of lattice girder, a series of specimens are designed and the mechanical influence of diagonal bar diameter and welding length on lattice girder are studied. The results show that: 1) The capacity of the lattice girder does not reduce with the decrease of the diagonal bar diameter, while the optimization index increases substantially. 2) The diameter of diagonal bar should meet the requirements of fabrication, storage, transportation and installation of lattice girder. 3) The reduction of the welding length, which can reduce the manufacturing cost of lattice girder, affects the capacity of lattice girder little.