This paper proposes a scheme which uses a single Mach-Zehnder modulator (MZM) to generate a flat and wide optical comb based on the double-sideband modulation method. The frequency difference between two lasers directly determines the spectral width and quality of this comb after double-sideband modulation. Through the theoretical analysis of the standard deviation between the sidebands of different orders under different modulation coefficients and the Bessel value of each order sideband, we find sidebands above the fifth order have a negligible effect on the wide comb. The lower-power fifth-order sidebands are superimposed on the sidebands of different orders, so that the powers of the sidebands of different orders can be compensated or cancelled, thereby reducing the influence of the higher-order sidebands on the optical frequency comb and improving the flatness. The flatness of the wide optical comb obtained by simulation reaches 3.7 dB, and the effective spectral width is 700 GHz. After the non-return to zero data with transmission rate of 5 Gbit·s^-1 is modulated on this optical comb and transmitted through an optical fiber, the eye diagram is still clearly visible, and the power penalty is less than 1.8 dB at the bit error rate level of 10^-9. Such an optical comb can transmit more data information in optical communications, realize high-speed data transmission and maintain the flatness of the optical comb with good stability and low cost.