In order to suppress the third-order crosstalk caused by the nonlinearity of the IQ modulator in the recirculating frequency shifter (RFS), and to improve the flatness of the multi-carriers and carrier to noise ratio (CNR), a modified RFS scheme based on linear IQ modulator (LIQM-RFS) is presented to generate multi-carriers. According to theoretical analysis, the range of driving voltage of linear IQ modulator (LIQM) radio frequency (RF) source which can be selected is twice of that of IQ modulator, with the premise of effective suppression of the third-order crosstalk. In particularly, the third-order crosstalk loss can still be repressed in a range where the first-order harmonic component of LIQM is shifted at a higher efficiency. With this feature, LIQM-RFS-based multi-carrier generator ensures the flatness of subcarriers, meanwhile it requires lower erbium-doped fiber amplifier (EDFA) gain. Therefore, the power of amplified spontaneous emission noise caused by the optical amplifier is lower, by which the CNR is improved. The simulation results show that CNR of multi-carrier generated by LIQM-RFS multi-carrier generator increases by about 9 dB compared to traditional RFS, and the subcarrier flatness of about 2.95 dB can be ensured.