This invention relates to a method for protecting conversion of backbone fibers in an EPON system, which includes: an optical circuit terminal with 1x2 optical switches, a working fiber and protection fiber, the method includes: finding trigger by fault or being triggered by a management event, utilizing 1x2 optical switch to switch the working fiber and th protection fiber to switch the connection of the normal state of the optical circuit terminal and the working fiber to the connection of the trrminal and the protection fiber.