A method for transporting a client signal in an optical transport network (OTN) includes steps as follows. A byte number Cn of a client signal transported in a OTN frame period is generated according to a client signal clock and a system clock. If the Cn of the OTN frame falls in a certain range, a predetermined area in an optical channel payload unit-k (OPUk) overhead field is identified as normal, and the Cn is filled in the OPUk overhead field of the OTN frame. Therefore, the reliability for transporting the client byte number can be improved and an OPUk overhead byte space needed for transporting the client signal byte number can be saved.