The relation between the deflection check coefficient (ηy) of mid-span and first vertical flexural frequency check coefficient (ηf) was established based on theory of the uniform cross-section simple-supported beam. The variation and difference between the ηy and 1/η2f of a simple-supported six girder bridge were researched by simulation under these situations: change of the bearing stiffness; change of the boundary conditions; the floor system enhanced the structural stiffness; local damage of the superstructure. The static loading test and dynamic characteristic test of two fabricated simple-supported T girder bridges were taken to get the measured ηy and 1/η2f. Simulation and experimental results show that 1/η2f can instead ηy to reflect the structural stiffness of the simple-supported girder bridge and rapid assess its load-bearing capacity.