Radiostrontium isotopes (89Sr and 90Sr) are important indicators for assessing radioactive contamination and radionuclide transport in marine environments. However, their determination in seawater remains analytically challenging because radiostrontium is a pure beta emitter and occurs at trace levels within a complex, high-salinity matrix. In this study, an automated radiochemical preconcentration and purification system was developed for the rapid pretreatment of radiostrontium from large-volume seawater samples, serving as a front-end module to interface with downstream radiation sensing applications. The automated radionuclide separator (ARS) platform integrates an ion-exchange chromatography (IEX) module for bulk Sr preconcentration, an extraction chromatography (EXC) module for selective Sr purification, and a LabVIEW-based digital-twin platform for automated process control. Resin amounts were optimized through breakthrough tests for both modules. The automated fluid-handling system showed excellent linearity between pump control voltage and flow rate (R2 = 0.9996) and high reproducibility. Under the optimized conditions, Sr was effectively retained and recovered with high yield while matrix carryover was reduced. The EXC module further demonstrated reproducible Sr recovery from the IEX elution solution, while Ba and Y were monitored as representative interfering elements for future selectivity optimization. These results demonstrate that the proposed automated system provides a reliable and reproducible pretreatment platform for radiostrontium analysis in large-volume seawater. The integration of digital-twin-based process control, stable fluid handling, optimized IEX preconcentration, and EXC purification offers a practical approach for rapid marine radioactivity monitoring and future scale-up toward onboard seawater analysis.