The utility model provides a stable tightener of a sewing machine and belongs to the technical field of machinery. The stable tightener is used for solving the problem the stable tightener of the existing sewing machine is unstable in thread clipping. The stable tightener of the sewing machine comprises a frame, a first hold-down piece and a second hold-down piece; the frame is arranged vertically and a supporting shaft is transversely and fixedly connected to the frame; the first hold-down piece and the second hold-down piece both are circular-ring-shaped; the first hold-down piece sleeves and is fixed on the supporting shaft, while the second hold-down piece sleeves the supporting shaft, and the positions of the first hold-down piece and the second hold-down piece directly face each other; the tightener further comprises a power mechanism capable of driving the second hold-down piece to translate back and forth; two grooves are formed in the end face, adjacent to the first hold-down piece, of the second hold-down piece, and the two grooves are located on the two sides of the supporting shaft, respectively; a convex sleeving portion is formed on the bottom wall of each groove, a spring is engaged with each sleeving portion, and one end of each spring extends out of the corresponding groove and props against the first hold-down piece. The stable tightener of the sewing machine has the advantage of stable thread hold-down.