Abstract: A chair with a synchronous coordinating system for the chair back includes a base, a chair cushion and a chair back. The base and the chair cushion are connected fixedly or formed integrally, and the chair back is articulated with the base which is articulated with the chair back through an upright column extended above the chair cushion. The articulated point is located at the trochanter (the centre point of the hip joint) of the human body. The chair cause the turning (pitched or inclined) pivot of the chair back and the human body on the same line by providing the articulated point of the chair back and the base at the trochanter of the human body in the pitching movement. No matter how the angle of the chair back is adjusted, the contact surface between the backside of the human body and the chair back will not be changed so that the chair back can provide sufficient supporting force to the waist at any angle and the person on the chair will not feel uncomfortable.