A clock-gating-based energy-efficient scheme is proposed, for the first time, for applications in optical network units (ONUs) accommodated by orthogonal frequency division multiplexing passive optical networks (OFDM-PONs) based on intensity modulation and direct detection (IMDD). In the operation of a conventional downlink OFDM-PON, each ONU has to perform the demodulation in physical layer for all the received OFDM frames, regardless the received frames belong to the ONU or not. To improve the ONU’s energy consumption efficiency, in this paper, frame identification and clock control modules are introduced into each ONU, where the former is to distinguish whether a received frame belongs to the ONU or not, and the latter is to control the operating clock of the OFDM demodulation module according to the frame identification output. As a result, when a non-local frame arrives, the operating clock is set to a low value by the clock control module to deactivate the OFDM demodulation module in order to avoid unnecessary power consumption of the module. Experiments are undertaken in a realtime IMDD OFDM-PON platform, and measured results show that 51% energy consumption of a field programmable gate array (FPGA) chip embedded in the ONU can be saved compared with its conventional counterpart for downlink unicast scenarios