Abstract: In this paper, the characteristics of roll stability of ship are introduced and analyzed, and a study method of stability moment when ship rolls in waves with a big inclination is presented. While calculating the immersed depth of the left and right side of transverse sections with step by step approaching method, the influences on the immersed depth of each transverse section due to the asymmetry between the fore and stern of ship , as well as that on roll stability by trimming during it rolls in waves are taken into account. Finally, the values of roll stability, in accordance with the condition of the balance between gravity and buoyancy of ship in waves and also with that the sum of trim moments is zero are obtained.