A service mapping processing method for an optical transport network, an apparatus, and a system are provided, where the method includes: generating mapping adaptation indication information according to a mapping granularity of a to-be-carried LO ODU, where the mapping granularity is M×g bytes, M is a quantity of timeslots occupied by the to-be-carried LO ODU in an OPUCn, g is a size of a mapping granularity corresponding to each timeslot of the timeslots occupied by the LO ODU, and g is a positive integer greater than 1; mapping, according to the mapping adaptation indication information, the to-be-carried LO ODU to an ODTUCn.M payload area; encapsulating the mapping adaptation indication information into the ODTUCn.M overhead area; encapsulating the ODTUCn.M into an OTUCn; and sending the OTUCn to a receive end device. The method avoids that OTUCns that use different mapping granularities cannot interwork between a receive end and a transmit end.