Spectral broadening and compression of a sub-terawatt (TW) laser pulse can be achieved by tightly focusing the pulse into a thin, dense gas target; in this way, the excited plasma wave drives self-phase modulation in the pulse and causes a coupled spatial-temporal evolution of field envelope. Through three-dimensional particle-in-cell simulations, selected focal positions of incident pulse, gas species, and target peak densities are assigned to investigate the performance of pulse compression. When a 0.25-TW, 40-fs, 810-nm pulse is incident into a hydrogen target with a 120-μm wide Gaussian density profile and a peak density of 8 × 10 19 cm-3, a shortest output duration of ≈ 20 fs is acquired when the pulse is focused to a size of 4 μm with a position 50 μm before the density peak. Under the same rest of parameters, using a nitrogen target inhibits the pulse compression due to undesired ionization-induced defocusing. Moreover, using a high peak density of 1.2 × 10 20 cm-3 for hydrogen target allows the 0.25-TW pulse to be self-focused to a high intensity capable of exciting a strong plasma wave, which, in turn, modulates and compresses the pulse to ≈7 fs, along with a significantly broadened spectral bandwidth ≈200 nm. This widely expanded spectrum supports a transform-limited pulse duration ≈2.8 fs and allows the output pulse to reach a TW-level peak power when appropriate post-compression is applied.