Alumina ceramics have high thermal conductivity, good heat dissipation, and a small dielectric constant. It is a common substrate material used in mobile communication and integrated electronics; however, its high hardness and brittleness make it crack easily during conventional machining. In this study, we utilized a nanosecond ultraviolet laser to study the blind cutting process of an ultra-thin ceramic plate. Moreover, we analyzed the slit width and depth variations by controlling the three main processing parameters of the nanosecond laser, which were laser repetition frequency, scanning speed, and repetition times. The experimental results show that as the laser repetition frequency increases, the slit width increases and remains unchanged, and the slit depth first increases and then decreases. As the scanning speed decreases and the number of repetition times increases, the slit depth first increases and then decreases, and the slit width slightly fluctuates. With the slit depth-width ratio as the index, we obtain the following conclusions through an orthogonal experiment: the maximum depth-width ratio of single-pass multiple scanning in the blind cutting process of the ultra-thin ceramic substrate is 4.0137, and the optimal laser parameter combination is 40 kHz laser frequency, 0.07 m/s scanning speed, and 25 repetition times.