A passive optical network PON system based on wavelength protection, comprising an Optical line terminal that includes a first interface circuit A (C1:int.1) and a second interface circuit B (C1: int.0), a first trunk fiber and a second trunk fiber, an optical splitter network that is connected with the first trunk fiber and the second trunk fiber, branch fibers that are connected with the optical splitter network, and multiple Optical network units (ONUs) P that include first interface circuits A' (C5 or C6: int.1) and second interface circuits B'(C5 or C6: int.0), characterized in that the passive optical network further comprises: a first WDM device, one end of which being connected with the first trunk fiber, and other end being connected with the first interface circuit A (C1:int.1) through a first interface optical fiber; a second WDM device, one end of which being connected with the second trunk optical fiber (C 14), and the other end being connected with the second interface circuit B (C1: int.0) through a second interface optical fiber; and multiple WDM devices (C4, C11) at Optical network unit side, one end of each being connected with one branch fiber (C10, C16), and the other end being respectively connected with the first interface circuit A' (C5: int.1) and the second interface circuit B' (C5: int.0) of one ONU P through two unit interface optical fibers (C17, C19); said first interface circuit A (C1:int.1) is adapted to transmit a downlink optical signal D A (λ1); the second interface circuit B (C1:int.0) is adapted to transmit a downlink optical signal D B (λ 3), wherein wavelengths of D A and D B are different (λ 1 # λ 3); said first interface circuit A' is adapted to transmit an uplink optical signal U A (λ2); the second interface circuit B' is adapted to transmit an uplink optical signal U B (λ4), wherein wavelengths of U A and U B are different (λ2 ≠ λ4); said first WDM device is adapted to pass the downlink optical signal D A (λ 1) and the uplink optical signal U A (λ 2), and refuse the uplink optical signal U B (λ 4), while said second WDM device is adapted to pass the downlink optical signal D B (λ3) and the uplink optical signal U B (λ4), and refuse the uplink optical signal U A (λ2); said optical splitter network is adapted to distribute optical power of downlink optical signals sent in through ports of the two trunk fibers to each port of the branch fiber, and optical signals sent in through each port of the branch fiber are super-positioned first and then output from ports of the trunk fibers; said WDM device (C4, C11) at Optical network unit side is adapted to respectively pass the downlink optical signals D A (λ1) and D B (λ3) that are transmitted by the branch fiber connected with it to the first interface circuit A' (C5: int.1) and second interface circuit B' (C5: int.0) of corresponding ONU P, and pass the uplink optical signals (λ 2 or λ 4) that are transmitted by the two interface circuits to said branch fiber. a first WDM device, one end of which being connected with the first trunk fiber, and other end being connected with the first interface circuit A (C1:int.1) through a first interface optical fiber; a second WDM device, one end of which being connected with the second trunk optical fiber (C 14), and the other end being connected with the second interface circuit B (C1: int.0) through a second interface optical fiber; and multiple WDM devices (C4, C11) at Optical network unit side, one end of each being connected with one branch fiber (C10, C16), and the other end being respectively connected with the first interface circuit A' (C5: int.1) and the second interface circuit B' (C5: int.0) of one ONU P through two unit interface optical fibers (C17, C19); said first interface circuit A (C1:int.1) is adapted to transmit a downlink optical signal D A (λ1); the second interface circuit B (C1:int.0) is adapted to transmit a downlink optical signal D B (λ 3), wherein wavelengths of D A and D B are different (λ 1 # λ 3); said first interface circuit A' is adapted to transmit an uplink optical signal U A (λ2); the second interface circuit B' is adapted to transmit an uplink optical signal U B (λ4), wherein wavelengths of U A and U B are different (λ2 ≠ λ4); said first WDM device is adapted to pass the downlink optical signal D A (λ 1) and the uplink optical signal U A (λ 2), and refuse the uplink optical signal U B (λ 4), while said second WDM device is adapted to pass the downlink optical signal D B (λ3) and the uplink optical signal U B (λ4), and refuse the uplink optical signal U A (λ2); said optical splitter network is adapted to distribute optical power of downlink optical signals sent in through ports of the two trunk fibers to each port of the branch fiber, and optical signals sent in through each port of the branch fiber are super-positioned first and then output from ports of the trunk fibers; said WDM device (C4, C11) at Optical network unit side is adapted to respectively pass the downlink optical signals D A (λ1) and D B (λ3) that are transmitted by the branch fiber connected with it to the first interface circuit A' (C5: int.1) and second interface circuit B' (C5: int.0) of corresponding ONU P, and pass the uplink optical signals (λ 2 or λ 4) that are transmitted by the two interface circuits to said branch fiber.