In this paper, a novel scheme is proposed and experimentally demonstrated. It is based on a directly modulated laser (DML) and all-optical mode-locking for generating tunable microwave frequency combs (MFCs). Theoretical analysis reveals that harmonic or rational harmonic mode-locking can be achieved by adjusting the parameters of the fiber ring cavity, which enables the generation of MFCs with adjustable comb spacing. Based on this, experimental verification shows that the DML can be driven to exhibit various typical dynamical states under sinusoidal modulation with different frequencies and amplitudes. These states serve as seeding signals that subsequently undergo all-optical mode-locking within the ring laser cavity, resulting in the generation of MFCs. The bandwidths of the MFCs are 13, 15, 19.5, 19.8, and 22 GHz, respectively, all of which satisfy the ±5 dB flatness criterion. A continuously tunable comb-spacing range of 200 MHz to 3 GHz is attained through the effective combination of the DML and all-optical mode-locking. The single-sideband (SSB) phase noise of the first comb line remains below –100 dBc/Hz at a 10 kHz offset. Theoretical analysis and experimental results demonstrate that the modulated signals of the proposed scheme support flexible parameter tuning over a wide range. Furthermore, the generated MFCs have remarkable advantages in flatness, bandwidth, and tunability.