A shower door with a concealed small-rail sliding assembly, comprising an upper rail assembly, a sliding assembly, a movable door assembly and a fixed door assembly, wherein the upper rail assembly comprises an upper rail and damper triggers mounted on the upper rail, wherein the upper rail is provided with a top plate, a first cover plate and a second cover plate which are parallel to each other extend downward from the two opposite sides of the top plate, and a bottom surface of the first cover plate is flush with a bottom surface of the second cover plate; wherein a first side sliding rail extends horizontally inward from an inner side of the first cover plate, a fixed door panel clamping slot is formed below the first side sliding rail, the bottom of the second cover plate is bent inward to form a horizontal second side sliding rail, and the bottom of the sliding rail surface of the first side sliding rail is higher than the sliding rail surface of the second side sliding rail; wherein a sliding cavity is defined by the top plate, the first cover plate and the second cover plate, and the whole sliding assembly is accommodated in the sliding cavity, thereby the sliding assembly is raised; wherein the sliding assembly comprises sliding wheels, sliding wheel brackets, a damper and a damper bracket, wherein the sliding wheels are mounted at two sides of each sliding wheel bracket, the sliding wheels on a first side of each sliding wheel bracket are in contact with the first side sliding rail, the sliding wheels on a second side of each sliding wheel bracket are in contact with the second side sliding rail, and the sliding wheels on the first side are higher than the sliding wheels on the second side; wherein the two sliding wheel brackets are respectively connected and fixed at the two opposite ends of the damper bracket, and the damper is accommodated and fixed in a damper accommodating groove of the damper bracket; and wherein the movable door assembly comprises a movable door panel and hanging clips fixed at a top edge of the movable door panel, the top edge of the movable door panel and the hanging clips are accommodated in the sliding cavity, and each sliding wheel bracket is connected with the corresponding hanging clip by an adjusting screw with the hanging clips on the movable door panel also being raised, so that the whole sliding assembly and the hanging clips are concealed in the sliding cavity. wherein the upper rail is provided with a top plate, a first cover plate and a second cover plate which are parallel to each other extend downward from the two opposite sides of the top plate, and a bottom surface of the first cover plate is flush with a bottom surface of the second cover plate; wherein a first side sliding rail extends horizontally inward from an inner side of the first cover plate, a fixed door panel clamping slot is formed below the first side sliding rail, the bottom of the second cover plate is bent inward to form a horizontal second side sliding rail, and the bottom of the sliding rail surface of the first side sliding rail is higher than the sliding rail surface of the second side sliding rail; wherein a sliding cavity is defined by the top plate, the first cover plate and the second cover plate, and the whole sliding assembly is accommodated in the sliding cavity, thereby the sliding assembly is raised; wherein the sliding assembly comprises sliding wheels, sliding wheel brackets, a damper and a damper bracket, wherein the sliding wheels are mounted at two sides of each sliding wheel bracket, the sliding wheels on a first side of each sliding wheel bracket are in contact with the first side sliding rail, the sliding wheels on a second side of each sliding wheel bracket are in contact with the second side sliding rail, and the sliding wheels on the first side are higher than the sliding wheels on the second side; wherein the two sliding wheel brackets are respectively connected and fixed at the two opposite ends of the damper bracket, and the damper is accommodated and fixed in a damper accommodating groove of the damper bracket; and wherein the movable door assembly comprises a movable door panel and hanging clips fixed at a top edge of the movable door panel, the top edge of the movable door panel and the hanging clips are accommodated in the sliding cavity, and each sliding wheel bracket is connected with the corresponding hanging clip by an adjusting screw with the hanging clips on the movable door panel also being raised, so that the whole sliding assembly and the hanging clips are concealed in the sliding cavity.