In order to investigate the process of molten material ejection during nanosecond laser ablation of mono-crystalline silicon, simulation and experimental investigation on nanosecond laser ablation of silicon under different environments were carried out. Simulation on the material ejection during nanosecond laser ablation of silicon under air, water, and vacuum environments with simulation software was carried out based on the Level-Set method. The effects of the temperature field and velocity field on surface sputtering under different environments were investigated. Experiments on laser ablation of mono-crystalline silicon using a wavelength of 266 nm, a pulse width of 30 ns, and a frequency of 50 Hz were done to verify the simulation results. The ablation results were observed by atomic force microscopy and digital microscopy. The results show that the spatter velocity under the air environment is 14.1 m/s at t=30 ns, and the molten material ejectes the ablated holes outwards due to the action of vapor pressure in the ablated area. Under the water environment, the spatter velocity is 1.68 m/s at t=30 ns, much lower than that in air conditions. Under the vacuum environment, the material is vaporized for a short time, and the spatter velocity of the molten material is 18.4 m/s at t=30 ns. The molten materials eject productively due to the high spatter velocity. The adhesion of molten material on the surface is improved. The processing environment has a strong influence on the material ejection during the nanosecond laser ablation of silicon. The investigation provides a reference for improving the quality during nanosecond laser micro-machining.