Defocusing amount and spatial distribution of laser energy have a significant effect on the quality of laser processing. It is a commonly used method in laser processing, and the method can adjust the lens distance between beam expanders to meet the different needs of defocusing amounts for the different laser processing materials. We use ZEMAX to model the optical system of laser processing with sequential and non-sequential modes. The beam expander is a Galileo transmission system composed of two lenses. In the simulation environment, the focal position and the spot energy distribution on the working plane are measured at different lens distances and tilt angles. The results show that the defocusing amount is proportional to the variation of lens distance, and the defocusing amount decreases with the decreasing distance. The negative defocusing amount increases with the increasing tilt angle. On the working plane, the peak power of laser spot decreases as the distance between the focal points of two lenses and the tilt angle increase. The effect of lens distance on the laser focusing characteristics is much greater than that of the tilt angle. The displacement of the laser spot is proportional to the tilt angle. The laser spot mode is always base-mode Gauss distribution. Finally, the defocusing amount and the spot energy distribution at different lens distances are measured. The experimental results are in good agreement with the simulation results.