The dynamic indentation fracture mechanics and particles impacting erosion theory were applied to the research of hydrodynamic suspension ultra-smooth machining. The theoretic model of nanoscale abrasive particles impacting workpiece surface micro asperities was established. Subsequently, the experiments of machining for aluminum alloy and K9 glass were carried out respectively. The results show that the hydrodynamic suspension machining is the process of nanoscale impacting the surface asperities to realize the micro removal, and the nanaoscale ultra-smooth surface without damage can be obtained during hydrodynamic suspension machining for K9 glass.