According to a double-thread lockstitch sewing method and a double-thread lockstitch sewing machine, through a sewing thread maintained by a needle, a sewing loop forming under a needle plate automatically forms a loop to realize thread-loosing prevention of stitches of double-thread lockstitch. Then by means driving of a movable knife, a sewing portion that projects on the back side of the sewing fabric is cut ; In addition to cutting in the vicinity of the needle drop position or needle drop position by the driving movement of the movable knife, a loop thread part protruding on the back side of the sewing fabric in a position close to the needle unit in a range that does not enter the vertical movement path of the needle was transported to the blade edge of the disposed fixed knife so as to cut the sewing portion through utilization of sliding contact between the loop thread part capturing part and an edge part of the fixed knife. Thus significant enhancement of sewing work efficiency is achieved without knot tying, other procedures and thread cutting working realized by scissors; and the yarn that protrudes on the back side of the sewing fabric is cut at proper length while not reducing anti-thread-loosing effects and ensuring the beautiful appearance of the sewing fabrics.