This invention discloses a kind of coaxial diaphragm loudspeaker unit that includes an electromagnetic loudspeaker. This electromagnetic loudspeaker includes a cone that is driven by a voice coil and a cone frame to secure the cone; a characteristic is its additional inclusion of a passive diaphragm; the said passive diaphragm is secured on the said cone frame using elastic components. It is coaxial with the said cone and has a ring-shaped circumference distributed outside it; at the same time, one side of the said passive diaphragm has a sealed connection to the air of one side of the said cone; the other side of the said passive diaphragm and the other side of the said cone are both exposed to the open space. These coaxial passive diaphragm and cone have uniform vibration with the air in the resonant cavity. This vibration is virtually unaffected by the internal geometric structure of the sealing cap. The coloration is also low so there need not be too many restrictions for the design of the sealing cap. It is only necessary to ensure sufficient sealing; the plan for the entire loudspeaker unit is simple and reliable, and it has good applicability.