The ultrafast ultraviolet(UV) laser source with central wavelength of 268 nm, spectrum width of 1.5 nm and single pulse energy of 0.58 mJ which is obtained by third harmonic generation of Ti:sapphire laser system with pulse duration of 70 fs and central wavelength of 808 nm can generate a plasma channel due to the nonlinear effect when it is injected into Ar gas cell after focusing. We studied the filament feature of UV laser at 268 nm and the modulation of spectrum by UV filament under the condition of different gas pressure, focal length and gas category. The spectrum width of 3.3 nm can be obtained with 2.2 atm Ar pressure and 1000 mm focal length which is a factor-of-2.2 improvement compared with input spectrum. We observed that both the increase of gas pressure and focal length are beneficial to the increase of plasma channel length and spectrum broadening. The plasma channel induced by UV pulses could provide a valid approach for attaining extremely ultrafast pulses in the UV range.