A framing method and apparatus in a passive optical network (PON) and a system, where the method includes generating a first transmission convergence (TC) frame and a second TC frame separately, wherein a sum of frame lengths of the first and the second TC frame is 125 microseconds (μs), performing bit mapping on the second TC frame to generate a third TC frame, where the bit mapping refers to identifying each bit of the second TC frame using N bits, and sending the first and the second TC frame to an optical network unit (ONU). A line rate corresponding to the second TC frame is lower than 2.488 giga bits per second (Gbps) such that a rate of a receiver on a receiving side is decreased and a bandwidth of the receiver is narrowed, thereby decreasing an optical link loss and increasing an optical power budget.