The invention discloses a wet electrostatic precipitator rear segment spraying program control optimized water saving method. The water saving method comprises the steps that firstly, a wet electrostatic precipitator circulating water quality solid containing amount online monitor is additionally arranged, so that the water quality of wet electrostatic precipitator circulating water is ensured; secondly, by automatically reading the current unit load and the spraying duration and period set at the corresponding load, the rear segment spraying water opening duration is automatically increased or decreased; thirdly, by means of newly increased spraying water amount calculated every 30 minutes, the drained waste water amount is synchronously calculated and adjusted, and wet electrostatic precipitator water balance is achieved; and fourthly, on the premise that the wet electrostatic precipitator circulating water quality solid containing amount meets the requirement, the amount of fresh water consumed by a wet electrostatic precipitator is reduced as much as possible, the water amount of an absorption tower is synchronously reduced, and therefore it is guaranteed that a demister of the absorption tower carries out flushing according to requirements, longer flushing duration is provided, the demisting effect of the system demister is ensured, blockage is avoided, the industrial water consumption is reduced to the maximum extent, and the drainage frequency and the drainage waste water amount of the absorption tower are synchronously reduced; and by means of the method, the wet electrostatic precipitator water consumption is reduced, and the method has a remarkable effect on economical and environmental-friendly running of a coal-fired unit.