In order to study the stress distribution effect of the webs which is under the action of vertical prestress, pairs of forces were acted on a long and narrow rectangular thin plate, and the webs stress field under the effect of vertical prestressing was simulated, with analytical solution of vertical normal stress derived and the numerical solution obtained by the Least-square method. The parameters,λ and κ, were introduced and calculated to describe vertical stress uniformity and stress level, which shows that λand κ value are more sensitive to webs vertical reinforcement spacing. On the premise of uniformity of vertical prestress and stress level, the fitting diffusion angle of vertical prestress acting on the webs is obtained and fitted by polynomial. Vertical prestress diffusion effect of the webs was tested in bridge site and analyzed by the finite element software. The results show that the webs stress diffusion is obvious, and the stress has a good uniformity at the section of H/2 but a bad uniformity at axillary and shore up.