Dynamic control of thermal emission is fundamental to radiative thermotronics and thermoregulation, yet precise spectral modulation has long been hindered by the intrinsic optical constraints of active materials. Conventional strategies rely on complex micro/nanostructures or multilayer metamaterials to induce optical resonances, which have severely restricted material compatibility and practical implementation. Here, we establish an optical phase-engineering principle for the spectral control of dynamic thermal emission. This is achieved by introducing a dielectric cap on top of metal-insulator transition (MIT) materials to create a highly tunable Fabry–Pérot cavity that enables a deliberate engineering of the reflection phase dispersion. As a result, this architecture transforms the broadband response of MIT materials into spectrally-selective dynamic emission. A Fresnel-based phasor analysis identifies the desirable reflected-wave phase profiles and material properties (high-index dielectric cap and low-loss metallic MIT state) that enable high spectral selectivity, which are further validated by Bayesian optimization. With these physical insights, we experimentally demonstrate a spectrally-selective emitter targeting the atmospheric transparency window (8–13 μm), with electrically tunable emissivity from about 0.2 to 0.9 through reversible copper electrodeposition on a germanium cavity. Furthermore, this strategy reconstructs the intrinsic optical responses of MIT materials in different wavelengths, enabling multispectral electrochromic switching between solar heating and selective radiative cooling. This work establishes a versatile paradigm for spectral engineering of dynamic thermal emitters with exceptional structural simplicity and universal material compatibility, opening broad opportunities for programmable radiative heat management.