Required by high-power, continuously tunable terahertz (THz) radiation from a room-scale source, a Linac-based beam injector was developed, which aimed at achieving a tradeoff between performance and cost efficiency. Under the overall consideration of compactness, design choices for the injector involving short bunch generation, energy boosting, as well as performance improvement were described in detail. A testing bench with online diagnostic elements was refitted from a prototype of such a Linac-driven THz radiation source, and measurement results indicated that such an injector showed an outstanding advantage in terms of system miniaturization without negatively affecting beam performance.