Reinforced concrete slab has the property of two-way bending. In this paper, four test specimens of reinforced concrete slabs were set up to be determined the strain and deflection at corresponding position. The results show that the bending of reinforced concrete slab does have two-way nature and the moment in one direction will cause the deformation of the other orthogonal direction, which can be reflected by the changes of curvature. At last, the conclusions are given: The calculation formula, which was derived from deflection calculation of beams and now is applied to the slabs, will result in no small calculation errors. The moment on one direction will cause the deformation of the other orthogonal direction and its size is related to the poisson’s ratio of the compressed concrete. And the size also is related to the strength grade of concrete and the reinforcement ratio, and so on. And the idea is further pointed out that it also can be used to solve the deflection calculation of concrete two-way slab.