In order to understand the influence of picosecond laser irradiation on crystalline silicon, laser-induced changes of crystalline silicon by picosecond laser ablation with different average laser powers were studied. Then X-ray photoelectron spectroscopy and transmission electron microscope were used to analyze the influence of average laser power variation on final chemical composition and microstructure of the ablated silicon, respectively. It is concluded that, with the increase of average laser power, the relative content of Si in ablation product keeps falling, by contrast, the relative content of SiO2 gradually rises. At the same time, the increase of average laser power intensifies the amorphization degree of microstructure of ablated silicon. Finally, it is deduced that the increased laser fluence resulted from average laser power increasing is the main reason for all of above experimental results, and also to enlarge and deepen the thermal and mechanical damages in ablated silicon.