An adjustable air stream introducing device for reinforcement of a blowing power of a primary air stream flowing through a duct, includes a tubular coupler disposed downstream of the duct to confine an annular space. A surrounding seat member is disposed securely in the annular space, and has a plurality of internal ports angularly displaced from one another. A surrounding sash member has a plurality of external ports, and overlies on and is turnable relative to the seat member via a motor-driven actuator member so as to adjust the overlying areas of the external and internal ports, thereby adjusting the amount of the introduced air stream through the annular space.