The sunflower-shaped arch bridge is a novel structure characterized by its main arch and web arch ring typically having a box-shaped section. Due to the influence of the web plate, the box-shaped section exhibits shear lag phenomenon when subjected to longitudinal bending. To investigate the shear lag effect of the arch ribs of sunflower-shaped arch bridges, this study analyzes the distribution patterns of shear lag effects for different sections under the most critical loading conditions using a specific sunflower-shaped arch bridge as a case study. The findings are compared with the results obtained using existing methods for calculating shear lag effects in box girder bridges. The results indicate that under different loading conditions, the distribution patterns of shear lag effects on the top plate of each section of the main arch are similar, with shear lag coefficients gradually increasing from the mid-span to the arch springing. However, the patterns of shear lag effects on the bottom plate of the sections are less distinct. The calculated values using the standard closely match the finite element analysis results for the mid-span section, but the standard calculated values are smaller than the finite element analysis results for the sections near the arch springing, and the calculated values according to standard is biased unsafe.