To clarify the distribution pattern of the stress field between vertical prestresses in large-span PC continuous box girder bridges and the layout criteria for the maximum vertical prestress spacing, an analytical solution for the web stress field under the action of two prestressed tendons was derived. On this basis, the distribution patterns of the vertical stress field under various prestress spacings and sectional heights were analyzed. Under the premise of ensuring a high-stress state in the web, this study proposed recommendations for the maximum prestress spacing by introducing the sectional stress uniformity index, addressing the blind zone of prestress induced by the vertical prestress diffusion angle. The specific results are as follows: ① The analytical solution of the stress field effectively reflects the distribution pattern of vertical normal stress in the web; ② The stress distribution among the vertical prestress tendons varies correspondingly with changes in the prestress spacing: When the vertical prestress spacing is relatively small, the diffusion effect is pronounced; as the vertical prestress spacing increases, the diffusion effect gradually transitions to a superposition effect; ③ For the control of vertical normal stress levels near the L/4 position of continuous box girder bridges, the maximum vertical prestress spacing should ensure that the stress uniformity level at the control section exceeds 0.95.