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Photonic-based low-phase-noise microwave generation with real-time frequency tuning is crucial for a broad spectrum of subjects, including next-generation wireless communications, radar, metrology, and modern instrumentation. Here, for the first time to the best of our knowledge, narrow-bandwidth dual-wavelength microlasers are generated from nearly degenerate polygon modes in a high-Q active lithium niobate microdisk. The high-Q polygon modes formation with independently controllable resonant wavelengths and free spectral ranges is enabled by the weak perturbation of the whispering gallery microdisk resonators using a tapered fiber. The stable beating signal confirms the low phase-noise achieved in the tunable laser. Owing to the high spatial overlap factors between the two nearly degenerate lasing modes as well as that between the two lasing modes and the pump mode, gain competition between the two modes is suppressed, leading to stable dual-wavelength laser generation and in turn the low noise microwave source. The measured microwave signal shows a linewidth of ~6.87 kHz, a phase noise of ~-123 dBc/Hz, and an electro-optic tuning efficiency of -1.66 MHz/V.