The plug- and- play structure is bidirectional and stable system in theory. However, the practical system can be attacked effectively due to the imperfect phase modulator. The modified phase- remapping attack is demonstrated. The minimal value of the bit error rate (BER) is derived, which is caused by the modified phase- remapping attack. The effect of the success rate of communication to the practical system is also considered. Under this attack, the BER can be kept the minimum value and a much better success rate of communication can be achieved.