The invention relates to a wavelength division multiplexing passive optical network multicasting system based on code superposition, belonging to the technical field of optical communication. The multicasting system comprises an optical line terminal, a feed-in line optical fiber, a remote-end code, a plurality of downstream distributing optical fibers and a plurality of optical network units, wherein the optical line terminal comprises a unicasting data signal generator and a multicasting data signal generator; and each of the optical network units comprises an optical powder divider, a unicasting data receiver and a multicasting data receiver. In the invention, the unicasting data is a signal encoded on the basis of a non-return-to zero code mode, the multicasting data is a signal encoded on the basis of a Manchester code mode, and the two signals are both signals based on an amplitude encoding mode, thus the system has very easy detection, less external interference and high stability, saves cost, and improves precision; and because the multicasting data adopts the Manchester code mode, the invention reduces power fluctuation in transmission, avoids the generation of long zero and long one, has good synchronization function and strong anti-interference capacity, and is very easy to recover clock signals at a receiving end.