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Terahertz (THz) spectrum holds immense potential for applications in chemical sensing, biomedical diagnostics, and security screening. However, conventional THz spectrometers rely on mechanical delay lines and coherent detection, posing major challenges for miniaturization, speed, and cost. Here, we report an active hybrid metasurface that functions as a low-correlation and ultrafast spectral/spatial light modulator for THz computational spectrometer and single-pixel imaging. Dispersion-engineered metasurface with multiple quasi-bound states in the continuum achieves dense high quality factor resonances across a spectral range (0.30-0.55 THz), enabling dynamic modulation through all-optical excitation of integrated silicon patches. Temporally modulated metasurfaces form a measurement matrix for incoherent single-pixel detection, allowing nanosecond-scale spectral reconstruction with a resolution of 0.03 THz covering a 0.25 THz bandwidth. Extending this platform to a pixelated metasurface array, we demonstrate ultrafast THz single-pixel imaging with 3×3 pixels. This CMOS-compatible, circuit-free strategy enables ultrafast, compact, and high-resolution THz spectroscopy and imaging, and offer a scalable pathway toward next-generation THz photonic devices.