Abstract: In this paper we proposed a scheme for asymmetric data transmission in Two-way Relaying Systems with physical layer network coding(PLNC).As we all known, the wireless links are discrepant. The sources nodes transmit message with asymmetric rates according to their individual link qualities by employing network coding at the relay. The scheme with asymmetric data transmission not only make full use of the better link to transmit more data, but also considerate the reliability in the worse link. The main idea to realize asymmetric data transmission is to add prior known information so as to diminish the constellation of the superimposed signal received in the relay and to get better performance of system in demodulation and decoding. The relay node joint demodulate the superimposed signal according to the constellation. Therefore, it can reduce the complexity of the communication system. Simulations show that the proposed transmission scheme can guarantee the reliability. In addition, it can improve the channel capacity of the system and take full advantage of the system resources.