The invention discloses an ONU, based on a directly-modulated laser, of a coherent PON system and a signal receiving method thereof, and relates to the ONU filed in the coherent PON system. A power divider of the ONU is provided with an uplink signal output end and a light source output end. A coherent receiver is provided with a downlink signal input end and a light source input end. An input end of the DML is connected with the power divider. The light source output end of the power divider is connected with the light source input end of the coherent receiver. An output end of the coherent receiver is connected with a signal recovery processor through a signal sampling processor. According to the invention, the manufacturing cost and power of the coherent ONU are lowered, and the influences of frequency jitter and phase noise of a local light source on signal quality are prevented. The advantages of the coherent passive optical network such as a large power budget, a high access speed and flexible channels are maintained, and the ONU of the coherent PON system is suitable for popularization.