The present invention provides a method, apparatus and system for transmitting and receiving a client signal. The method for transmitting a client signal includes, at the transmitting end, mapping a client signal to be transmitted to a corresponding low-order Optical Channel Data Unit (ODU) in a low-order ODU set, wherein low-order ODUs in the low-order ODU set having rates increased in order, and having rate correspondence relations with the client signals; mapping the low-order ODU to a timeslot of a high-order Optical Channel Payload Unit (OPU) in a high-order OPU set; and adding overheads to the high-order OPU to form an Optical Channel Transport Unit (OTU), and transferring the OTU to an Optical Transport Network (OTN) for transmission.