The utility model provides an automatic cutting device behind single broken yarn of two -for -one twisting, doubling all -in -one. The utility model discloses a frame, a yarn section of thick bamboo is installed to installation mated twisting dish in frame lower part on every twisting dish, and every yarn section of thick bamboo top is provided with a first cord roller, and is every to being provided with two second cord rollers side by side above the twisting dish to two second cord roller tops of row are provided with a coiling roller, first cord roller and the yarn between the second cord roller on hang and have two asymmetrical U type cutters, the horizontal inboard length of asymmetrical U type cutter equal two distances between the yarn, the length that has the long side arm of blade is greater than two distances between the yarn, the length of the short side arm of opposite side is not more than 23, two of two distances between the yarn the installation opposite direction of asymmetrical U type cutter. The utility model discloses when the fracture appeared in a yarn, another yarn of automatic cutout was avoided single yarn to remain in operation and is formed yarn waste's the condition.