A ball screw assembly comprising: a guider having a central axis and a plurality of spiral channels, wherein the spiral channels are 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 a plurality of inner spiral channels, the inner spiral channels correspond to the spiral channels of the guider, and the inner spiral channels and the spiral channels form a plurality of inner ball races; a first circulator (40, 40a) disposed on the first axial wall, wherein the first circulator (40, 40a) comprises a plurality of first curves (41, 41a), and two ends of each of the first curves (41, 41a) are respectively in communication with the corresponding inner ball race and the corresponding outer ball race; a second circulator (50, 50a) disposed on the second axial wall, wherein the second circulator (50, 50a) comprises a plurality of second curves (51, 51a), and two ends of each of the second curves (51, 51a) are respectively in communication with the corresponding inner ball race and the corresponding outer ball race; and a plurality of ball circulating assemblies (60, 60a) each comprises a plurality of balls, wherein one of the inner ball races, one of the first curves (41, 41a), one of the outer ball races, and one of the second curves (51, 51a) form a first ball circulating race, another one of the inner ball races, another one of the first curves (41, 41a), another one of the outer ball races, and another one of the second curves (51, 51a) form a second ball circulating race; one of the ball circulating assemblies rolls (60, 60a) in the first ball circulating race, and another one of the ball circulating assemblies (60, 60a) rolls in the second ball circulating race, characterized in that a plurality of open shields (30, 30a) each coaxially fitted over an outer periphery of the axial cylinder, wherein each of the open shields (30, 30a) has an inner peripheral wall (31, 31a), the inner peripheral wall (31, 31a) corresponds to the outer annular wall of the axial cylinder, and the inner peripheral wall (31, 31a) 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, each of the outer ball races is perpendicular to the central axis. a guider having a central axis and a plurality of spiral channels, wherein the spiral channels are 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 a plurality of inner spiral channels, the inner spiral channels correspond to the spiral channels of the guider, and the inner spiral channels and the spiral channels form a plurality of inner ball races; a first circulator (40, 40a) disposed on the first axial wall, wherein the first circulator (40, 40a) comprises a plurality of first curves (41, 41a), and two ends of each of the first curves (41, 41a) are respectively in communication with the corresponding inner ball race and the corresponding outer ball race; a second circulator (50, 50a) disposed on the second axial wall, wherein the second circulator (50, 50a) comprises a plurality of second curves (51, 51a), and two ends of each of the second curves (51, 51a) are respectively in communication with the corresponding inner ball race and the corresponding outer ball race; and a plurality of ball circulating assemblies (60, 60a) each comprises a plurality of balls, wherein one of the inner ball races, one of the first curves (41, 41a), one of the outer ball races, and one of the second curves (51, 51a) form a first ball circulating race, another one of the inner ball races, another one of the first curves (41, 41a), another one of the outer ball races, and another one of the second curves (51, 51a) form a second ball circulating race; one of the ball circulating assemblies rolls (60, 60a) in the first ball circulating race, and another one of the ball circulating assemblies (60, 60a) rolls in the second ball circulating race, characterized in that a plurality of open shields (30, 30a) each coaxially fitted over an outer periphery of the axial cylinder, wherein each of the open shields (30, 30a) has an inner peripheral wall (31, 31a), the inner peripheral wall (31, 31a) corresponds to the outer annular wall of the axial cylinder, and the inner peripheral wall (31, 31a) 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, each of the outer ball races is perpendicular to the central axis.