In a service signal processing method, an optical network unit (ONU) receives a service signal; maps the service signal to a flexible optical service unit frame; and sends a first passive optical network transmission convergence frame to an optical line terminal (OLT), where the flexible optical service unit frame is encapsulated in the first passive optical network transmission convergence frame, and where the flexible optical service unit frame is used to carry the service signal in a passive optical network (PON) and an optical transport network (OTN). In this application, the flexible optical service unit frame can be transmitted in both the PON and the OTN, and the ONU and the OLT do not need to parse the service signal.