This paper introduces a new box girder widening method- Steel cantilever widening concrete box girder method without piers (SCWCGM for short). Without additional piers, SCWCGM can still meet the need of widening range, which is an effective widening method for urban bridges. In this paper, the widening conception and the structure style of SCWCGM is detailed. In order to study the mechanical properties of this method, the full-scale model test was conducted. The results show that, the SCWCGM is reasonable and feasible.