A compound needle for a flat knitting machine, comprising a main needle and a needle sleeve capable to sleeve the main needle; the main needle is elongated, and a needle hook is disposed at a head portion of the main needle, wherein a direction in which the needle hook is bent is defined as a right side, and a direction opposite to the direction in which the needle hook is bent is defined as a left side; two planes of the main needle perpendicular to the right and left sides are defined as front and back sides; the main needle is provided with a lug boss that protrudes outwards from the right side and is positioned below the needle hook; an inwards-recessed accommodating groove is respectively formed in front and rear sides of the lug boss; the left side of the main needle is bent inwards to form an inwards-recessed make-way groove, wherein, a top end of the needle sleeve positioned to the hook side of the main needle is provided with an apex angle which is of a hollow structure consisting of two side sheets adapted to wrap the needle hook; an open slot structure composed of the two side sheets is formed in one side of the needle sleeve facing the main needle; a first salient point is formed on one of the side of the main needle facing to the needle sleeve or the side of the needle sleeve facing to the main needle, and a trajectory surface is formed on the respective other side; wherein the first salient point cooperates with the trajectory surface such that when the needle sleeve moves, the apex angle moves from the accommodating grooves of the main needle to close the needle hook of the main needle, and the front and rear sides of the main needle are recessed inwards to form a concave surface (16, 17), and the needle sleeve is composed of an intermediate sheet and the two sides sheets; the apex angle at the top end of the needle sleeve consists of the two side sheets; characterized in that one side of each of the two side sheets of the needle sleeve facing the main needle extends outwards to form two clamping strips; and the corresponding clamping strips on the two side sheets form the first fastener and the second fastener; the needle sleeve is clamped on the concave surface of the main needle through the open slot by the clampings strips forming first fasterners and second fasteners, respectively; portions of the side sheets exceeding the intermediate sheet forming two sidewalls of the open slot structure, and a middle section of each of the the two sidewalls extending between the respective upper end and the respective lower end, is provided with a gap; each of the first fastener and the second fastener are formed at an upper end and a lower end of each of the two sidewalls; each of the two side sheets extending outwards towards one side of the main needle so that the first fastener and the second fastener are oppositely bent; the concave surface (16, 17) on each of the front side and the rear side of the main needle consists of a first concave surface and a second concave surface which are arranged in sections; the first fastener and the second fastener are respectively fastened on the first concave surface and the second concave surface of the main needle; and an elastic sheet capable to be pressed by the needle groove is disposed on one side of the needle sleeve facing away from the main needle, and is of an elongated broken line shape.