A method for transmitting a client signal in an optical transport network, OTN, comprising: acquiring a client signal, and computing a client signal byte number Cn transported in an OTN frame period according to a client signal clock and a system clock; and characterized in that the method further comprises: saving a range of the Cn of a detected client signal transported in one OTN frame period, wherein the computed Cn falling in the range represents that a client signal type of the acquired client signal is the same as a client signal type of the detected client signal; in a first area of a Cbyte field of an optical channel payload unit-k, OPUk, identifying whether the computed Cn exceeds the range, wherein the Cbyte field is divided into the first area and a second area; if the computed Cn exceeds the range, filling in a second area of the Cbyte field of the OPUk with the computed Cn; and if the computed Cn falls in the range, filling in the second area of the Cbyte field of the OPUk of the OTN frame with a Cn filled in a previous OTN frame, wherein, if the Cn transported in the OTN frame needs to be increased, reversing values of a first series of bit positions of the second area; if the Cn transported in the OTN frame needs to be decreased, reversing values of the second series of bit positions of the second area; in the frame next to the OTN frame, mapping the computed Cn client signal bytes to an OPUk payload area, and transmitting an optical channel transport unit, OTU, comprising the OPUk to the OTN network. acquiring a client signal, and computing a client signal byte number Cn transported in an OTN frame period according to a client signal clock and a system clock; and characterized in that the method further comprises: saving a range of the Cn of a detected client signal transported in one OTN frame period, wherein the computed Cn falling in the range represents that a client signal type of the acquired client signal is the same as a client signal type of the detected client signal; in a first area of a Cbyte field of an optical channel payload unit-k, OPUk, identifying whether the computed Cn exceeds the range, wherein the Cbyte field is divided into the first area and a second area; if the computed Cn exceeds the range, filling in a second area of the Cbyte field of the OPUk with the computed Cn; and if the computed Cn falls in the range, filling in the second area of the Cbyte field of the OPUk of the OTN frame with a Cn filled in a previous OTN frame, wherein, if the Cn transported in the OTN frame needs to be increased, reversing values of a first series of bit positions of the second area; if the Cn transported in the OTN frame needs to be decreased, reversing values of the second series of bit positions of the second area; in the frame next to the OTN frame, mapping the computed Cn client signal bytes to an OPUk payload area, and transmitting an optical channel transport unit, OTU, comprising the OPUk to the OTN network. saving a range of the Cn of a detected client signal transported in one OTN frame period, wherein the computed Cn falling in the range represents that a client signal type of the acquired client signal is the same as a client signal type of the detected client signal; in a first area of a Cbyte field of an optical channel payload unit-k, OPUk, identifying whether the computed Cn exceeds the range, wherein the Cbyte field is divided into the first area and a second area; if the computed Cn exceeds the range, filling in a second area of the Cbyte field of the OPUk with the computed Cn; and if the computed Cn falls in the range, filling in the second area of the Cbyte field of the OPUk of the OTN frame with a Cn filled in a previous OTN frame, wherein, if the Cn transported in the OTN frame needs to be increased, reversing values of a first series of bit positions of the second area; if the Cn transported in the OTN frame needs to be decreased, reversing values of the second series of bit positions of the second area; in the frame next to the OTN frame, mapping the computed Cn client signal bytes to an OPUk payload area, and transmitting an optical channel transport unit, OTU, comprising the OPUk to the OTN network.