The present disclosure discloses a thread tension adjusting device comprising a body, a tension rod, an elastic element and an adjusting knob. A thread passing eye for inputting a thread is arranged below the body. A tension wheel and a thread passing wheel are rotatably arranged on the body. An adjusting knob is used for adjusting a rotational speed of the tension wheel. The elastic element has one end connected to the body and another end connected to the tension rod. The thread tension adjusting device may ensure stability of tensioning and transmitting of the thread by cooperation between the tension rod and the tension wheel, and improve precision for thread tension adjusting by the adjusting knob. In addition, the thread tension adjusting device may monitor thread breakage by the tension detector, and ensure operating stability and safety of the thread tension adjusting device.