Embodiments of the present invention provide a method and an apparatus for controlling a transmit power of an optical network unit, and an optical network unit. The method includes: obtaining passive optical network identification information; obtaining a transmit power of an optical transmitter of an optical line terminal and minimum sensitivity of an optical receiver of the optical line terminal according to the passive optical network identification information; obtaining a receive power of an optical receiver of the optical network unit; calculating line attenuation according to the transmit power of the optical transmitter of the optical line terminal and the receive power of the optical receiver of the optical network unit; and adjusting a transmit power of an optical transmitter of the optical network unit according to the line attenuation and the minimum sensitivity of the optical receiver of the optical line terminal. According to the method, the apparatus, and the optical network unit in the embodiments of the present invention, an upstream optical transmit power of the optical network unit can be dynamically adjusted, so that energy consumption is reduced.