Abstract: The principle of tacking stitching of lockstitch sewing machine was analyzed,pointing out that whether the tacking stitching is in good order or not lies in how to control the tacking relay.Based on the characteristic of the lockstitch sewing machine,there are two method can be used.Both of them begin from one of needle signals of the sewing machine.The first one is that when the needle signal arrives,a delay subroutine will be called and then the tacking relay act at once.The second one is that when the needle signal arrives,a position pulse count subroutine will be called and then the tacking relay act at once too.The experimental results are presented in the end,which indicate that the tacking stitching is much orderly when the second method was used.