Using Gaussian decomposition (GD) method, we studied the theory of Z-scan with large nonlinear phase shift induced by a pulsed laser. It has been verified that the GD method is still valid to deal with analysis of Z-scan measurements with large nonlinear phase shift. By comparing the peak-valley configuration of the Z-scan curves for large nonlinear phase shift induced by pulsed and CW laser, we found that some new peak-valley features of the Z-scan curves appear as the aperture size or light intensity increases in the case of large nonlinear phase shift. Meanwhile, we carried out the Z-scan experiments of pure CS2 to confirm the results of numerical simulation in the case of large nonlinear phase shift induced by a picosecond pulsed laser. The experimental results agree well with the theoretical. Our results have some significance to the measurement of Z-scan with large nonlinear phase shift induced by a pulsed laser.