Design and implementation of a realtime transport cascade multiplexer in SDTV are discussed. The main devices of the system are DSP and FPGA. It can multiplex multiple SDTV programs into one TS stream through cascade of multiple multiplexers. Setting up a higher code rate, many null packets in TS stream can be generated by upper multiplexer, so local generated TS packets can be inserted into location of these null packets by cascade multiplexer. Through multiplexer cascade, multiple SDTV programs can be multiplexed into one TS stream.