The internal-force distributing of the slab computed according to the existing design methods is different from that of the bidirectional slab supported on beam. The mechanical characteristic of the slab on unbonded prestressing tendons concentrated arranged along the columns is analyzed. The results show that the hidden beams of prestressing tendons couldn’t replace the role of the actual beam. Therefore, the prestressing tendons layout in the beam-less slab should not be disposed according to the internal stress distribution of the two-way slab supported on actual beams. Based on the FEM, a accurate method for calculating the amount of two-way prestressing tendons is put forward according to the bearing counterforce that existe in beam-supported two-way slab. The linear of the prestressing tendons is deduced and reasonable value of the equivalent load of prestressed tendons is suggested.