Abstract: To meet the ever increasing applications of femtosecond laser pulse, it has to be measured more precisely. Based on the spectral phase interferometry of direct electricfield reconstruction (SPIDER), the spectral phase of femotosecond laser pulse was reconstructed with numerical simulations. The crucial parameters that include the time delay τ between the tested pulse replicas, the frequency shear Ω and the width of filter window were analyzed to choose their values properly. With the Gauss linearly chirped pulse whose full width at half maximum (FWHM) is about 20fs, the phase is reconstructed with different time delay and different frequency shear Ω/Δω. The optimum phase curve is about at τ=1210fs and relative shear is Ω/Δω=0.09. The width of filtering window is about τ/3.