A ball 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 ball race; wherein in a virtual plane projection of the open nut from a radial direction of the open nut, the outer ball race is perpendicular 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 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 ball race; and a ball circulating assembly comprises a plurality of balls, wherein the inner ball race, the first curve, the outer 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 annular wall of the axial cylinder form the outer 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 ball race; wherein in a virtual plane projection of the open nut from a radial direction of the open nut, the outer ball race is perpendicular 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 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 ball race; and a ball circulating assembly comprises a plurality of balls, wherein the inner ball race, the first curve, the outer 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 annular wall of the axial cylinder form the outer ball race.