In order to investigate the geometrical factors that influence the surface deflections and the influence rule, a box shaped part with a triangular step surface was designed. The simulation model was built and validated, and sheet forming experiments and simulations were carried out to study the influence of the geometrical factors on surface deflections of automobile exterior panels. The results indicate that the geometric size factors, including the sharp angle, radius of the sharp corner, depth of the step surface, and radii at the up and down steps, will seriously affect the surface deflections of the box shaped part with a triangular step. However, different geometric factors show different laws about the influence. At the same time, it is noted that depth of the step surface and radius of the sharp corner are the two most sensitive factors that influence the surface deflection by orthogonal tests. The results of this work will play a guiding role on the control of surface deflection of automobile exterior panels in term of the design and optimization of products.