Continuous assembled box girder bridges are increasingly used in bridge engineering due to their high degree of assembly, short construction period, and high smoothness and comfort of traveling. To grasp the transverse force performance of this type of bridge structure, this paper, based on the industry standard drawings, adopts the method of finite element numerical analysis to systematically study the effects of different miter angles, number of diaphragm girders, span diameters, and width-to-span ratios on the transverse distribution coefficients of the loads and conducts the field test on the real bridge. The results show that the lateral distribution of main girder loads decreases with the increase of miter angle, the increase of the number of diaphragm beams, and the increase of span diameter, and increases with the increase of width-to-span ratio; the field static load tests were carried out on the three bridges, and the measured lateral distribution patterns of the bridge loads match well with the theoretical analysis of the finite element theory. The transverse distribution coefficients of the main girders obtained from the field test are reasonable, indicating that the force distribution of the transverse connection between the main girders is reasonable. The analysis of the transverse force of the assembled small box girder bridge can initially determine whether the bridge integrity is good or not, and provide technical data support for the subsequent operation and maintenance of the bridge.