The invention provides an air cooling system of an oil-immersed transformer. The air cooling system is characterized in that the air cooling system includes a fan M and three-phase power source lines; the three-phase power source lines are connected with a knife switch ZK and a relay KM; the relay KM is provided with three sets of normally-open contacts; the relay KM is connected with a fuse FR; the fuse FR is connected with the fan M; and control loops for controlling the fan are connected between two-phase lines of the three-phase power source lines. With the above technical schemes of the invention adopted, air cooling operation or an air cooling disappearance signal is reported to a main control room background, so that an operator can monitor an air cooling state in the background, and conveniences can be brought for the operator in air cooling state monitoring.