Abstract: Finite element method is adopted to investigate the difference of the mobility of thin plates with different stiffened beams, boundary conditions and sizes of loading areas, and the criterion of adopting surface mobility, which is related to the frequency of loading, is obtained. The difference of area mobility and point mobility of a beam-stiffened plate is investigated to reveal the criterion whether "point" and "area" can be equivalent. Further research on the type of stiffened beams and loading area as well as the distribution of the velocity of thin plates reveals that among the plate without beam, the single beam-stiffened plate and the cross beam-stiffened plate the criterion of adopting surface mobility for the plate without beam is the most strict, while that of the cross beam-stiffened plate is the least strict. Then the distribution of the velocity of a single beam-stiffened plate is calculated which validates the correctness of the criterion of adopting surface mobility for the single beam-stiffened plate. Finally, the differences of mobility of single beam-stiffened plate with different shapes of loading area are analyzed, which indicate that if the shape of the loading area is rectangle, when two parallel edges of the rectangle are perpendicular to the stiffened beam and became longer and the other two parallel edges of the rectangle became shorter, the difference between the point mobility and surface mobility of single beam-stiffened plate will be larger.