Abstract: The course route of elastic-plastic bending capacity during different phases is studied in order to investigate the mechanics properties of plain masonry bearing walls which carry bending moments and invariable axial forces under a horizontal earthquake action. The formulas for bending capacity at each stage of different circumstances of initial cracking or crushing are theoretically deduced on the basis of a constitutive relationship assumption under uneven stress and stress figure equalization on a normal section. Furthermore, programming and numerical simulating are carried out. The researches indicate that normal section cracks firstly under a usual value of axial compressive stress to strength and bending capacity will not degenerate later but increase gradually, and normal section crushes and bending capacity degenerates linearly after ultimate bending capacity is reached; bending cracking begins at local edges and gradually expands, meanwhile bending capacity exhibits some kind of ductility similar to that of a reinforced concrete wall or column; the course of tensile cracking or compressive crushing becomes shorter along with the increase of compression ratios. On the other hand, the results indicate that normal section crushes firstly under the higher value of compression ratios and course route curves of bending capacity show the trend of a continuous decline.