Aball screw assembly comprising a guider having a central axis and a spiral channel, wherein the spiral channel is annularly disposed around an outer periphery of the guider; an open nut slidably fitted over the guider, wherein the open nut comprises an axial cylinder, the axial cylinder has an axial opening, the axial cylinder has a first axial wall and a second axial wall respectively located at two opposite sides of the axial opening; the axial cylinder comprises an inner annular wall and an outer annular wall, the inner annular wall has an inner spiral channel, the inner spiral channel corresponds to the spiral channel of the guider, and the inner spiral channel and the spiral channel form an inner ball race; an open shield coaxially fitted over an outer periphery of the axial cylinder, wherein the open shield has an inner peripheral wall, the inner peripheral wall corresponds to the outer annular wall of the axial cylinder, and the inner peripheral wall and the outer annular wall form an outer spiral ball race; wherein in a projection of a virtual plane of the open nut, the outer spiral ball race is obliquely configured with respect to the central axis, the virtual plane is perpendicular to a symmetrical plane defined on the open shield upon the open shield is observed from one end of the central axis, and the virtual plane is parallel to the central axis; a first circulator disposed on the first axial wall, wherein the first circulator comprises a first curve, and two ends of the first curve are respectively in communication with the inner ball race and the outer spiral ball race; a second circulator disposed on the second axial wall, wherein the second circulator comprises a second curve, and two ends of the second curve are respectively in communication with the inner ball race and the outer spiral ball race; and a ball circulating assembly comprises a plurality of balls, wherein the inner ball race, the first curve, the outer spiral ball race, and the second curve form a ball circulating race, and the ball circulating assembly rolls in the ball circulating race; characterized in that: the inner peripheral wall of the open shield has an inner channel, the outer annular wall of the axial cylinder is a curved surface, and the inner channel and the outer peripheral wall of the axial cylinder form the outer spiral ball race. a guider having a central axis and a spiral channel, wherein the spiral channel is annularly disposed around an outer periphery of the guider; an open nut slidably fitted over the guider, wherein the open nut comprises an axial cylinder, the axial cylinder has an axial opening, the axial cylinder has a first axial wall and a second axial wall respectively located at two opposite sides of the axial opening; the axial cylinder comprises an inner annular wall and an outer annular wall, the inner annular wall has an inner spiral channel, the inner spiral channel corresponds to the spiral channel of the guider, and the inner spiral channel and the spiral channel form an inner ball race; an open shield coaxially fitted over an outer periphery of the axial cylinder, wherein the open shield has an inner peripheral wall, the inner peripheral wall corresponds to the outer annular wall of the axial cylinder, and the inner peripheral wall and the outer annular wall form an outer spiral ball race; wherein in a projection of a virtual plane of the open nut, the outer spiral ball race is obliquely configured with respect to the central axis, the virtual plane is perpendicular to a symmetrical plane defined on the open shield upon the open shield is observed from one end of the central axis, and the virtual plane is parallel to the central axis; a first circulator disposed on the first axial wall, wherein the first circulator comprises a first curve, and two ends of the first curve are respectively in communication with the inner ball race and the outer spiral ball race; a second circulator disposed on the second axial wall, wherein the second circulator comprises a second curve, and two ends of the second curve are respectively in communication with the inner ball race and the outer spiral ball race; and a ball circulating assembly comprises a plurality of balls, wherein the inner ball race, the first curve, the outer spiral ball race, and the second curve form a ball circulating race, and the ball circulating assembly rolls in the ball circulating race; characterized in that: the inner peripheral wall of the open shield has an inner channel, the outer annular wall of the axial cylinder is a curved surface, and the inner channel and the outer peripheral wall of the axial cylinder form the outer spiral ball race.