We study the nonreciprocal transmission of a single-photon in a cavity optomechanical system, in which the cavity supports a clockwise and a counter-clockwise circulating optical modes, the mechanical resonator (MR) is excited by a weak coherent driving, and the signal photon is made up of a sequence of pulses with exactly one photon per pulse. We find that, if the input state is a single-photon state, it is insufficient to study the nonreciprocity only from the perspective of the transmission spectrums, since the frequencies where the nonreciprocity happens are far away from the peak frequency of the single-photon. So we show the nonreciprocal transmission behavior by comparing the spectrums of the input and output fields. In our system, we can achieve a transformation of the signal transmission from unidirectional isolation to unidirectional amplification in the single-photon level by changing the amplitude of the weak coherent driving. The effects of the mechanical thermal noise on the single-photon nonreciprocal transmission are also discussed.
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