In order to study the transverse distribution change of hollow core slab bridge after reinforcement by transversal steel plate, a simply supported hollow core slab bridge was employed as an example. The field load test data of the bridge before and after reinforcement were compared with the theoretical results by hinged-joint slab method and modified rigid-joint slab method. It is found that the phenomenon of ‘single plate load effect’ can be clearly displayed by comparing the measured and calculated values of the transverse distribution ratios. As the trend line formed by the measured data was not smooth, the relative transverse distribution growth value was introduced in the paper. Although the cast-in-situ deck and transverse steel plate reinforcement enhanced the transverse stiffness, the measured transverse distribution ratios were in good agreement with the calculated values according to the hinge-jointed slab method. The results show that the theory of hinge-jointed slab method can meet the transverse distribution calculation before and after the strengthening of such bridges.