Pulse-width expansion of an ultra-short Gaussian pulse in nonlinear optical crystal because of low order group velocity dispersion (GVD) was theoretically analyzed. Optical parametric interaction processes of the CsLiB6O10 (CLBO) crystal with a negative uniaxial and 50 fs ultra-short Gaussian pulse have been considered. Mechanism of ultra-short Gaussian pulse broading with or without initialization chirp modulation was numerically calculated. The pulse-width is expanded heavier in ordinary light than that in extraordinary in short wavelengths. The shorter pulse wavelength, the heavier pulse-width expanded. When the crystal length is equal to the crystal dispersion length, the pulse-width has been expanded 2 times than the initialization. Also, the shorter the dispersion length is, the more obviously the pulse-width is expanded.