We have proposed a slit-type bridge restrainer as a possible method to retrofit a steel highway bridge to prevent the fall-off of girders. The slit type restrainer provides enough moving distance and energy absorption capability to comply with the revised specification of highway bridges. These characteristics critically depend on the large plastic deformation of the restrainer plate. In this work, we examine the mechanical behaviors of the slit type restrainer by parametric FEM analyses with the establishment of practical design method in mind. The numerical analysis also covers the possible application of new type material (low yield point steel) and the estimation of ductile fracture.