Abstract: The layout of steel reinforcement in the mid-span and the support of the rectangular concrete slab with four simply supported sides are usually based on the elastic thin plate theory for small deformation. This paper studies the distribution of torsion moment in the slab based on the same theory. The results show that the distribution of the torsion moment in the slab is quite complex. The magnitude of the torsion moment at the corner of the slab is close to the sagging moment in the mid-span. The combined moment consequently tears up the corner of the slab. The current design code for the simply supported rectangular slab only considers the constitutional reinforcement. Therefore, it does not satisfy the requirement for baring the torque at the corner of the plate and the reinforcement should be provided at the corner. From the comparison of the moment distribution along the diagonal line in simply supported rectangular plate and the fixed-end plate, this work demonstrates that the restraint of a curb beam for the plate can help relieve the cracking of concrete at the corner of the plate. The insufficiency of the current code in the design of simply supported floor slab is pointed out, and the corresponding recommendations in the design of one-way and two-way slabs are also proposed in this paper.