A penetrating electric toothbrush is disclosed, including a handle, a replaceable brush head, a head unit base, an engaging device, a selection device, and an actuator. The replaceable brush head comprises a head body, a plurality of bristles fixed to the head body, and a detachable fastener. The head unit base has a head receptacle portion, a actuator receiving recess, and a bar guiding portion. The reciprocating bar is protruded from the handle and received in the actuator receiving recess, and the actuator receiving recess is configured to engage the reciprocating bar in either penetration mode or polishing mode. The engaging device is disposed at an end of the reciprocating bar and engaging the head unit base with the reciprocating bar. The selection device places the engaging device either in the penetration mode or the polishing mode.