Abstract: A covert communication system based on the transform domain communication system (TDCS) is proposed. The covert signal is superimposed on the original satellite communication signal and a transparent transponder is used for covert communication. First, the system's transmitting and receiving processes are analyzed, and the information hiding model is established. Based on this secure paradigm of information hiding, the security of the overlay covert communication is defined. Finally, the capabilities and the security of the system are simulated and analyzed. The simulation results show that TDCS has excellent anti-jamming ability. The bit error rate(BER) of the system is about 10^-4, even when the jamming-to-signal ratio is 15 dB and the signal-to-noise ratio is -13 dB. TDCS is more suitable for the overlap covert communication system compared with the direct sequence spectrum spreading. The overlapped covert signal has little effect on the data received by the original system, and sothe overlap covert communication system can be considered to be secure.