Slip phenomenon on a (001) silicon surface under 1064 nm laser ablation was studied by experiments and simulations. The surface morphologies of the silicon wafers after laser irradiation were observed using an optical microscope. The slip patterns showed that slip that occurred before melting was responsible for a low-quality ablation surface. The slip damage was predicted by a three-dimensional finite element model based on heat transfer and thermoelasticity theory. The judging criterion of slip was explained in detail. The numerical results gave a better understanding of slip phenomenon in experiments. It is shown that low laser irradiances cause slip and high laser irradiances are helpful in preventing slip. The threshold irradiance is ∼1 MW/cm2. Lasers with higher irradiance are essential to obtain a slip-free ablation on a (001) silicon surface.