The rear surface ablation damage phenomenon is investigated in the process of picosecond laser ablation cutting of K9 glass. The ablation qualities on the front and rear surfaces are compared, and the influences of laser fluence, scanning times and scanning speed, etc., on the rear surface damage are studied. Based on the ablation morphology and quality on the rear surface, the laser fluence, scanning speed, and other parameters at the cutting condition of single line and single pass are optimized. The ablation and damage mechanism of rear surfaces is discussed and the potential approach to suppress the rear surface damage is proposed. Experimental results show that, when the laser beam is focused on the front surface and the ablation cutting occurs, the rear surface is seriously ablated and damaged, and the morphology and ablation rule on rear surface are significantly different from those on front surface.