An approach to generate a flat optical comb with tunable comb spacing and adjustable comb number is proposed. In the proposed approach, a Mach-Zehnder modulator (MZM), being biased to generate two carrier-suppressed first-order sidebands, is cascaded with a phase modulator. The two optical sidebands are then sent to the phase modulator to generate two identical, but frequency-shifted phase-modulated spectra. Thanks to the complementary nature of the two adjacent comb lines in the phase-modulated spectra, the overlapping of the two spectra would lead to the generation of a flat optical comb. Since only the phase modulation index or the microwave power is needed to be adjusted, the system is easy to be implemented with tunable comb spacing and adjustable comb number. Numerical simulations are performed, and the approach is verified by an experiment.