This article describes a terahertz (THz) pulse slicer composed of two laser-driven semiconductor switches (LDSSs) to generate high-power, nanosecond-scale narrowband THz pulses. A tunable LDSS with a double semiconductor wafer structure improves the isolation and reduces the insertion loss. The phase shift of the THz waves propagating within the LDSS can be precisely controlled by adjusting the distance between the wafers for destructive interference of the reflected waves with near-zero off-state reflection across a broad frequency range and reduced propagation loss. The LDSS achieves better than17.5 dB isolation and less than 0.15 dB insertion loss with 4 ns response time.