The invention provides an automatic online measurement system and method for high-concentration ions. The system comprises a volume metering unit, a diluting unit, a dilution water supplying unit, a fluid drainage unit, an online ion analyzer and a measurement and control unit. The method comprises the steps that sampling is performed, and volume metering is performed on a solution to be measured through the volume metering unit; a first valve is opened, so that the solution to be measured in the volume metering unit after volume metering is drained into the diluting unit; the dilution water supplying unit supplies water to the volume metering unit so that the volume metering unit can be washed, and washing fluid obtained through washing enters the diluting unit; after the surface of the fluid in the dilution container calms down, the fluid level is measured through a fluid level sensor; the dilute solution in the dilution container is placed into a fluid storage container; the concentration of the ions to be measured in the dilute solution is measured through the online ion analyzer; the concentration of the ions to be measured in the solution to be measured is acquired through the measurement and control unit. The automatic online measurement system and method have the advantages that measurement accuracy is high, the concentration measurement range is wide, and the system and method can adapt to solutions under the working conditions of high temperature, high corrosivity and easy crystallization.