The invention provides a method for spinning high-grade knitting yarns by using long flax fibers. The method comprises the steps of: carrying out crystallization modification treatment on the long flax fibers, carrying out gilling, combing, slivering, drawing, roving, boiling-off, bleaching and spinning operations sequentially, and carrying out electronic yarn clearing and waxing simultaneously in a spooling process, thereby forming the knitting yarns. According to the method provided by the invention, the crystallization modification treatment is carried out on the long flax fibers, so that the fibers are soft in texture and have an improved uniformity for dry adjustment, therefore a foundation for spinning the high-grade knitting yarns is laid; and electronic yarn clearing and waxing operations are simultaneously carried out in the spooling process owing to the facts that the fibers are soft in texture and have an improved uniformity for dry adjustment after the crystallization modification treatment is carried out on the long flax fibers, therefore the spinning efficiency is improved, and the knitting yarn quality is improved.