A cube unit deformation decomposition method meeting complete orthogonality and mechanical equilibrium conditions comprises the first step that spatial deformation of a 8-node cube unit is composed of rigid body displacement in the X direction, rigid body displacement in the Y direction, rigid body displacement in the Z direction, tension and compression deformation in the X direction, tension and compression deformation in the Y direction, tension and compression deformation in the Z direction, X-axis bending deformation in an XOY plane, Y-axis bending deformation in the XOY plane, Y-axis bending deformation in a YOZ plane, Z-axis bending deformation in the YOZ plane, X-axis bending deformation in an XOZ plane, Z-axis bending deformation in the XOZ plane, shear deformation in the XOY plane, shear deformation in the YOZ plane, shear deformation in the XOZ plane, rigid body rotation displacement in the XOY plane, rigid body rotation displacement in the YOZ plane, rigid body rotation displacement in the XOZ plane and X-axis reverse bending deformation. Based on the method, an engineering designer can conveniently decompose ductile deformation (such as tension and compression deformation and bending deformation) and brittle deformation (such as shear deformation and torsion deformation) of a structure according to finite element analysis and laboratory test and field test data.