The invention provides an energy-saving cooling device of a direct air cooling system. A water spraying device communicated with desalted water in a desalted water tank is additionally mounted on a rising pipeline of a steam exhaust pipe of a turbine low pressure cylinder. An air cooling system disclosed by the invention can be used for directly cooling inlet air of an air cooling condenser in conditions of higher environment temperature and unit operation with high back pressure, so that the purposes of energy conservation, consumption reduction and unit output improvement are achieved by reducing the back pressure of the unit. According to the technology disclosed by the invention, by additionally arranging the atomizing and water spraying device in the rising pipeline of the steam exhaust pipe of a turbine, the desalted water is atomized and introduced into the inlet air of the air cooling condenser, so that the heat load of the air cooling condenser is reduced and the unit operation vacuum is improved.