In order to simplify the analyses of thin walled open profile bars, a thin walled bar was split into two parts which were dominated by in plane and out plane loading effects respectively. The in plane loading effects were focused. On the basis of appropriate simplified stress and strain conditions, each plate was analyzed and the results were integrated into vectors, resulting in the slabs' in plane tension bending resistance system of thin walled open profile bars, which reflects their axial stretch/compression, bending and warping properties. And then the deformation equation and the stiffness equation were set up. Because longitudinal interaction forces between plates were introduced in the definition of the plates' in plane bending moments, the deduction and conclusions here were consistent in form with those in bending theory. Principal axes' directions, shear center's coordinates, principal inertia moments and sectorial inertia moment of thin walled open sections can be deduced by the slabs disassembled method proposed here. Lateral deformed bars' analysis based on those sectional parameters in classical theory is proved to be inefficient, where additional “rigid contour hypothesis” has to be introduced.