A picosecond laser with a high repetition rate of MHz is used to ablate stainless steel 304 and the surface after ablation is observed and measured by laser scanning confocal microscope (LSCM) and scanning electron microscope (SEM). The results show that the ablation rate is jointly determined by the single pulse peak fluence and the repetition rate. The ablation rate increases as the ratio of pulse peak fluence to ablation threshold increases. Better ablation quality of surfaces can be achieved under the conditions of lower peak fluence and higher repetition rate. With the increment of scanning speed, the ablation depth decreases logarithmically, and the roughness degree of ablation surface decreases first and then stabilizes. When the scanning speed is between 1.0 m/s and 1.7 m/s, the ablation rate shows no obvious change. The use of cross scanning path is beneficial for improving the surface quality after ablation.