The ramp B of one overpass was designed as curved steel orthotropic box girder, whose deck was welded with open shaped bulb ribs in the range of driveways. Then, one detailed finite element model was built to simulate the structure. For the two arrangements of bulb towards and back to web in the cantilever, the tangential stresses of diaphragm cutout were compared under the action of vehicle load. In accordance with above research, it can be concluded that the stress will be more reasonable with the bulb back to web. After altering the transverse position of the vehicle load and calculating the tangential stress along the edge of diaphragm cutout, based on further analysis, it’s generally believed that the following two reasons made the arrangement of bulb back to web more appropriate at least. First of all, the diaphragm connected with the rib adjacent to the web could share part of vertical load. What's more, it could increase the distance from the edge of the cutout to the web center. What come next was to change the diaphragm cutout size, and that parameter optimization was carried out. The results show that a radius of 40-50mm is more applicable for the arc on the bottom of the diaphragm cutout.