The present disclosure relates to a light attenuation controlling apparatus for a CDM lamp, comprising: a full-bridge inverter as a power source to supply an output to the CDM lamp; a driving circuit for driving the full-bridge inverter; a single-chip microcomputer connected to the driving circuit; a tube voltage sensing module, one end of which is connected to the full-bridge inverter so as to sense a tube voltage currently outputted by the full-bridge inverter to the CDM lamp, and through the other end of which the tube voltage of the CDM lamp is outputted to the single-chip microcomputer. The present disclosure provides a novel light attenuation controlling apparatus for a CDM lamp, which can effectively control light attenuation of the CDM lamp by constantly regulating the output of the full-bridge inverter, thereby enhancing duration of the CDM lamp.