The main characteristic of WDM-PON is the node-tree structure. In order to preciously locate faults in each branch, a method based on tunable chaotic Fabry-Perot laser is proposed. Chaotic light emitted from a optical feedback multiple-longitudinal mode Fabry-Perot semiconductor laser diode is utilized as the probe beam. By selecting the feedback mode, a tunable chaotic laser is obtained. The branches of the optical networks are distinguished by the laser′s wavelength, and fault location is realized by calculating the cross-correlation of transmitted and back-reflected signals. In the experiment, we analyse the chaotic property and take a 1×4 ITU WDM-PON to be detected, the measurement results show that the breakpoints and connectors could be preciously located, an rang-independent spatial resolution within 4 cm are achieved with this method.