An optical network failure recovery method for a passive optical network is provided. The method includes steps of: obtaining at least one round-trip delay value according to an equalization delay value of at least one optical network unit with respect to an operating optical line terminal; sorting the round-trip delay values in an increasing order and obtaining a minimum round-trip delay value; obtaining at least one difference between the minimum round-trip delay value and the round-trip delay value of each of the optical network units; and adjusting a start time and a stop time in a bandwidth map for a ranging response message replied by each of the optical network units according to the at least one difference.