A flexible flat optical frequency comb (OFC) generator based on a single integrated polarization-multiplexing dual-drive Mach–Zehnder modulator (PM-DMZM) and a single RF source is proposed and demonstrated. Only one PM-DMZM is required, which guarantees the simplicity. Due to the additional controllable parameter of the polarization as compared with conventional schemes, flat coherent OFCs can be obtained with relatively small modulation indices. A theoretical model is established and an experiment is carried out. Five-, 7-, 9-, 11- and 13-line OFCs with flatness of 0.5, 0.7, 1.2, 1.65, and 2.7 dB are experimentally demonstrated, respectively.