Thread cutting device for sewing machine

The utility model relates to a thread cutting device for a sewing machine, comprising a base, an auxiliary member positioned on the base via a locking element, and a cutting knife group penetrated into the base. The cutting knife group is provided with a connecting member and a thread cutting knife member; the connecting member is connected with a driver of a sewing machine; a slide seat is connected with the auxiliary member in a sliding way and fixed on the connecting member, and the thread cutting knife member is fixed on the slide seat via a fixing element with adjustable tightness. When the connecting member is driven by the driver, the auxiliary member is closed with the thread cutting knife member to form a knife edge to do thread cutting operation.