The invention provides an automatic thread cutting device in a multi-needle machine, which belongs to the technical field of machines. The invention solves the technical problems of low guide precision of a movable blade, poor stability of thread cutting and the like in the traditional thread cutting devices of multi-needle machines. The automatic thread cutting device arranged at the needle headof the multi-needle machine comprises a needle plate pad and a movable blade holder arranged on the needle plate pad, wherein a fixed blade is fixed on the needle plate pad and a plurality of movableblades are fixed on the movable blade holder. The movable blade holder is connected with a driving mechanism, a guide mechanism which enables the movable blade holder to slide in the plane direction under the driving of the driving mechanism is arranged on the needle plate pad, and the sliding of the movable blade holder enables the movable blades to be in a position tangental to the fixed blade.The automatic thread cutting device ensures that the movable blade holder is more stable and precise in sliding, and enhances the stability of thread cutting.