The theoretical model of type-Ⅰ and type-Ⅱ phase matching conditions for second harmonic generation, group velocity delay of fundamental and second harmonic, and limitation of nonlinear crystal matching length by group velocity mismatch, was obtained by using energy conservation and three-wave coupled equations in optical parametric process of ultra-short pulses. The relation of the group velocity matching length of the harmonic and fundamental wavelength is analyzed by numerical simulation for a negative uniaxial crystal CsLiB6O10 with an 100 fs pulse. The maximum group velocity matching length of nonlinear crystal is 19.1 mm for fundamental wavelength 642 nm in type-Ⅰ and 0.89 mm for wavelength 767 nm in type-Ⅱ, which correspond to the minimum group velocity delay time, respectively.