The disclosure provides a method and system for injecting cutting fluid during milling under different working conditions. By analyzing influence of an airflow field in a milling area under different working conditions on injection of cutting fluid, an influence rule of a helical angle and a rotation speed of a cutter on the flow field is quantitatively analyzed, an optimal target distance of a nozzle, an angle between the nozzle and a milling cutter feeding direction and an angle between the nozzle and the surface of a workpiece are comprehensively determined, the nozzle is set according to a determined setting manner, and lubricating oil is sprayed to the milling area by utilizing the nozzle.