The output quality of frequency-locked multi-carrier source based on recirculating frequency shifter (RFS) technology is easily influenced by the inherent high-order harmonic crosstalk of modulator, especially the third-order crosstalk. In order to reduce the third-order crosstalk, a scheme that another RF signal whose frequency is 3fm is loaded on both ports of I/Q modulator is adopted. The first-order signal it produces is used to suppress the third-order crosstalk. A multi-carrier source whose number is 24 and maximal power difference is smaller than 0.1 dB is achieved through theoretical analysis and simulation research. The effective optical signal-to-noise ratio (OSNR) of multi-carrier source is improved by 2 dB compared with that before using the scheme of suppressing third-order crosstalk.