Abstract: A ranging method and system for passive optical network, and an optical network unit are provided. The ranging method includes: sending an initial ranging signal to a splitter loop-back device and recording a current sending time as a first time; receiving a loop-back ranging signal from the splitter loop-back device, and determining whether the loop-back ranging signal is a return signal of the initial ranging signal; if the loop-back ranging signal is a return signal of the initial ranging signal, recording a current receiving time as a second time; and obtaining ranging information by calculating a difference between the first time and the second time, and performing a ranging on the basis of the ranging information.