A primary lens for a vehicle headlight, comprising a main body portion , which comprises a first optical surface and a second optical surface , wherein a straight focus line is formed as a peak portion on the first optical surface , and the heigth of the first optical surface is gradually reduced from the focus line towards two sides; and the second optical surface comprises a plurality of ridges arranged in parallel and each of the ridges extending in a direction perpendicular to the focus line , and the ridges have arc-shaped convex faces, wherein a sub-optical surface is formed respectively at each side of the focus line and comprises a plurality of convex arc-shaped surfaces, characterized in that the radii of the arc-shaped surfaces are increased sequentially in a direction from the focus line to either side in each of the sub-optical surfaces, and the plurality of arc-shaped surfaces are sequentially connected smoothly, wherein each of the plurality of convex arc-shaped surfaces is a circumferential surface of a cylinder with a central axis parallel to the focus line , wherein, when the primary lens is applied to the headlight, the focus line extends in the horizontal direction and the first optical surface causes incident light to converge toward the focus line in a vertical direction, wherein a first direction, a second direction and a third direction are regarded as directions of three axes of a three-dimensional rectangular coordinate system, the first direction is a front-rear direction of a vehicle, the second direction is a left-right direction of the vehicle, and the third direction is the vertical direction. first optical surface and a second optical surface , wherein a straight focus line is formed as a peak portion on the first optical surface , and the heigth of the first optical surface is gradually reduced from the focus line towards two sides; and the second optical surface comprises a plurality of ridges arranged in parallel and each of the ridges extending in a direction perpendicular to the focus line , and the ridges have arc-shaped convex faces, wherein a sub-optical surface is formed respectively at each side of the focus line and comprises a plurality of convex arc-shaped surfaces, characterized in that the radii of the arc-shaped surfaces are increased sequentially in a direction from the focus line to either side in each of the sub-optical surfaces, and the plurality of arc-shaped surfaces are sequentially connected smoothly, wherein each of the plurality of convex arc-shaped surfaces is a circumferential surface of a cylinder with a central axis parallel to the focus line , wherein, when the primary lens is applied to the headlight, the focus line extends in the horizontal direction and the first optical surface causes incident light to converge toward the focus line in a vertical direction, wherein a first direction, a second direction and a third direction are regarded as directions of three axes of a three-dimensional rectangular coordinate system, the first direction is a front-rear direction of a vehicle, the second direction is a left-right direction of the vehicle, and the third direction is the vertical direction. <EOS>