According to the theory of multi-carrier generation based on recirculating frequency shifter (RFS), a modified scheme for multi-carrier generation to improve signal to noise ratio (SNR) through noise suppression by inserting an asymmetric Mach-Zehnder interferometer (AMZI) in the RFS loop is proposed creatively. The formulas of optical signal power and amplified spontaneous emission (ASE) noise power after AMZI are deduced. According to the assumption that the AMZI arms optical path difference is much shorter than the coherence length of the laser and longer than the coherence length of the ASE noise, ASE noise is reduced by 3 dB without affecting the optical signals power simultaneously. 50 output subcarriers are generated with both simulation and experiment confirmations, respectively. The results show that carrier to noise ratio of the multi-carrier generated by the improved scheme can be as high as 37.5 dB and is up to 17.5 dB higher than the traditional structures.