A method for accessing a Passive Optical Network (PON) is disclosed. The method includes: obtaining a first distance parameter of an Optical Network Unit (ONU), where the first distance parameter indicates the location of the ONU relative to an Optical Line Termination (OLT); receiving an ONU information request message delivered by the OLT, and returning an ONU information response message to the OLT when a second distance parameter carried in the ONU information request message corresponds to the first distance parameter; and receiving a ranging request message, where the ranging request message is delivered by the OLT according to the ONU information response message, returning a ranging response message as a basis for the OLT to obtain an Equalization Delay (EQD) value of the ONU, and connecting the ONU to the PON according to the EQD value. An ONU, an OLT, and a PON are also disclosed. The embodiments of the present invention enable the ONUs located at different distances from a same PON port to access the PON in lossless mode, thereby making full use of port resources and ensuring efficient running of devices.