Long-term online monitoring of metals in surface water is essential to protect water resources. Herein, a novel, integrated analytical system encompassing an automatic sample preparation system and solution cathode glow discharge-optical emission spectroscopy (SCGD-OES) for continuous and real-time detection of metals in rivers is presented. A piston pump integrated with a switching valve in the SCGD-OES system enables on-site preparation of water samples and controlled injection of a standard solution with a gradient concentration or environmental samples directly into the SCGD unit. Corresponding hardware and software platforms were developed to support automated operation. Under laboratory conditions, the achieved limits of detection (LODs) for typical metal ions, e.g., Na, K, Ca, and Mg, are comparable to those of conventional SCGD setups. The evaluation of the SCGD system using certified reference materials (CRMs) for water samples demonstrated satisfactory performance for elemental quantitative analysis, with relative errors of 4.1%, 1.9%, 4.0% and 4.5% for the four elements Na, K, Ca and Mg respectively. Furthermore, a seven-day field deployment of the SCGD system at a riverine site confirmed its accuracy and stability, with a relative error of less than 5.7% relative to reference ICP-OES measurements. These results validate the ability of the SCGD system to provide reliable and continuous online monitoring of metals in surface water.