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.