This research investigates the parallel cutting process of silicon wafer using a green picosecond laser, spatial light modulator, and GS feedback algorithm. It examines the influence of several factors such as the overlap ratio of adjacent beams in multiple beams, defocus magnitude of beams, and the energy of single pulses from defocused beams on the depth and flatness of slits. The results show that the laser overlap ratio should be controlled between 70% ~ 80%, which can maximize the single cutting depth and make the bottom of the cutting groove flat. The positive defocus of the array beam is helpful to improve the flatness of the bottom of the cutting groove. The defocus amount is determined by the Rayleigh length of the focused beam and the beam aberration. Under these experimental conditions, optimal results are yielded when the defocus quantity is maintained at 1 to 2 times the Rayleigh length. After the multi-beam is defocused, the cutting depth is reduced. In a certain range, the cutting depth can be effectively improved by increasing the single pulse energy of the defocused beam in the multi beam without affecting the flatness of the bottom of the cutting groove.