Using density functional theory with local spin density approximation and taking SrRuO₃ as an example, the crystal structure of ABO₃ perovskite oxide and the rectangular deformation of the BO₆ octahedron were studied. It was found that the degree of octahedral rectangular deformation of orthorhombic SrRuO₃ is much larger than Jahn–Teller distortion, therefore, cannot be ignored. The lattice constant a is greater than b in SrRuO₃ (as well as LaCrO₃, LaGaO₃, and PbRuO₃) because the side length of rectangle along x direction, a_oct, is larger than that along y direction, b_oct; and the tilting angle of the octahedron, φ_T, is smaller than arccos (b_oct/a_oct). We found that in ABO₃ oxide, BO₆ octahedral rectangular deformation has a universality and, consequently, a_oct ≠ b_oct. We discussed in detail the changes of lattice constants caused by rectangular deformation of the octahedron and divided orthorhombic ABO₃ oxides into the following three categories: a_oct ​> ​b_oct with a > b; a_oct ​> ​b_oct with a < b; and a_oct ​≤ ​b_oct with a < b. The relationship between the rectangular deformation and the rotation (tilting) angle or tolerance factor is analyzed. As the symmetry and lattice constants of SrRuO₃ are rather inconsistent, we have analyzed and clarified them. The influence of rectangular deformation of the octahedron on the electronic structure and magnetic properties of SrRuO₃ was also calculated using first principles.