Most currently available terahertz (THz) narrowband filters contain a metal and a substrate, which introduce absorption loss and spectral fluctuations caused by a Fabry-Perot interference in substrates. To address these issues, we employ quasi-bound states in the continuum (BICs) for the design and realization of a substrate-free all-dielectric THz transmissive narrowband filter. Under oblique incidence, the symmetry-protected BICs break and collapse into high-Q transmissive quasi-BIC modes, thereby achieving narrowband filtering. The filter not only minimizes energy loss but also demonstrates a smooth filtering response without an interferential spectral fluctuation associated with the substrate. An experimental high Q value of ∼127 at 4.1 THz with a broad sideband of ∼1.5 THz with transmittance below 10% is achieved.